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LIBRARY  OF 

Dr.  CARL  F.  W.  BODECKER 

1846-1912 

The  gift  of 

Dr.  Henry  and  Dr.  Charles  Bodecker 

1929 


DENTAL  METALLURGY 


A 


Manual  for  the  Use  of  Dental 
Students. 


BY 

CHAS.  J.  ESSIG,  M.D.,D.D.S., 

PROFESSOR     OF    MECHANICAL    DENTISTRY    AND    METALLURGY    IN    THE 
DENTAL  DEPARTMENT   OF   THE    UNIVERSITY   OF   PENNSYLVANIA. 


THIRD  EDITION,  REVISED. 


PHILADELPHIA: 
The  S.  S.  White  Dental  Mfg.  Co 

1893. 


Copyright. 

The  S.  S.  White  Dental  Manufacturing  Co. 

1882. 

Copyright  by  Chas.  J.  Essig,  1887. 

Copyright  by  Chas.  J.  Essig,  1S93. 

ALL  RIGHTS   RESERVED. 


PRESS  OK 

PATTERSON  &  WHITE, 

PHILADELPHIA. 


\ 


PREFACE  TO  THE  THIRD  EDITION. 


IN  the  revision  of  this  work  for  a  third  edition,  the 
author  has  adopted  the  system  of  spelling  and 
pronunciation  of  chemical  terms  as  recommended  by 
the  Chemical  Section  of  the  American  Association 
for  the  Advancement  of  Science,  among  the  promi- 
nent features  of  which  is  the  dropping  of  the  final  e 
from  all  words  terminating  in  ide,  the  pronunciation 
being  id,  as  chlorid,  iodid,  etc.  ;  and  in  sulphur  and 
its  compounds  the  spelling  is  changed  to  sulfur,  sulfid, 
sulfite,  sulfate,  sulfo,  etc. 

The  work  has  not  been  greatly  enlarged,  the 
author  believing  that  for  the  present  its  original  scope 
is  sufficient  for  the  needs  of  dental  students. 

The  chapters  on  Amalgams,  Aluminum,  and  Iron 

and  Steel  have  been  somewhat  amplified,  and  made 

to  embody  the  recent  improvements  in  the  production 

of  those  materials. 

Charles  J.  Essig. 
Nov.  6,  1893. 


PREFACE  TO  THE  SECOND  EDITION. 


THE  practical  results  of  the  publication  of  the  first 
edition  of  the  Manual  of  Metallurgy,  then  the 
first  text-book  of  the  kind  adapted  to  the  use  of 
dental  students,  showed  that  it  filled  a  want  long 
felt,  and  demonstrated  the  necessity  for  preparing  a 
second  edition.  I  have  accordingly  carefully  revised 
the  work,  and  while  it  has  not  been  greatly  enlarged, 
the  more  recent  improvements  in  the  reduction  of 
metals  and  the  formation  of  alloys  and  amalgams 
used  in  dentistry  have  been  incorporated.  As  in  the 
first  edition,  my  aim  has  been  to  avoid  extraneous  or 
merely  hypothetical  matter,  as  well  as  that  belonging 
to  the  purely  chemical  study  of  the  metals,  his  knowl- 
edge of  which  it  is  presumed  the  student  should 
derive  from  other  sources. 

The  success  of  the  first  edition,  in  so  far  as  it 
has  been  of  service  to  students  of  dentistry,  and  the 
kindly  reception  accorded  it  by  my  collaborators, 
are  subjects  of  peculiar  gratification  and  grateful 
appreciation  on  my  part. 

Charles  J.  Essig. 


PREFACE  TO  THE  FIRST  EDITION. 


THE  object  of  the  author  in  the  preparation  of  this 
Manual  was  to  place  in  the  hands  of  students 
of  dentistry  an  outline  of  the  scientific  principles  in- 
volved in  the  reduction  of  the  metals,  their  properties, 
the  modifications  resulting  from  alloying,  and  their 
application  to  dental  uses. 

While  the  properties  of  many  of  the  metals  have 
been  only  incidentally  or  illustratively  referred  to, 
special  consideration  has  been  given  to  those  most 
commonly  used  by  dentists. 

In  Chapter  V  a  resume  of  the  author's  experiments 
in  the  formation  of  alloys  for  amalgams  is  given. 
These  were  made  for  the  purpose  of  enabling  him  to 
present  the  subject  systematically  to  the  students  who 
sat  under  his  teachings  ;  and,  while  the  chapter  is  far 
from  being  a  complete  treatise  on  the  subject,  it  is 
hoped  that  it  will  prepare  the  dental  student  for  a 
better  comprehension  of  that  branch  of  metallurgy, 
occupying,  as  it  does,  so  conspicuous  a  place  in  the 
practice  of  dentistry. 

As  the  atomic  weights,  specific  gravity,  and  fusing- 
points  of  the  metals  are  somewhat  differently  stated 
by  various  authors,  the  figures  given  in  this  Manual 

5 


6  PREFACE   TO   THE    FIRST    EDITION. 

have  been  made  to  correspond  to  those  in  Fownes's 
Chemistry, — the  text-book  commonly  used  by  dental 
students. 

The  decimal  system  has  been  used  to  express  pro- 
portions, in  the  belief  that  its  comprehensiveness  and 
simplicity  would  commend  it. 

In  expressing  temperatures  the  Centigrade  and 
Fahrenheit  scales  have  been  occasionally  referred  to 
separately.  The  rule  for  translating  one  into  the 
other,  and  comparative  tables,  have  been  omitted 
because  they  are  to  be  found  in  Fownes's  and  nearly 
all  the  other  recent  works  on  chemistry,  with  some 
one  of  which  the  student  is  supposed  to  be  more  or 
less  familiar.  C.  J.  E. 


CONTENTS. 


CHAPTER 

I.     Metallurgy   . 

II. 

The  Metallic  Elements     . 

III. 

Properties  of  the  Metals 

IV. 

Alloys 

V. 

Amalgams        .... 

VI. 

Modes  of  Melting  Metals 

VII. 

Combinations  of  Metals  with 

tallic  Elements     . 

VIII. 

Gold 

IX. 

Silver 

X. 

Platinum 

XL 

Iridium     . 

XII. 

Palladium 

XIII. 

Iron 

XIV. 

Mercury  . 

XV. 

Copper     . 

XVI. 

Zinc 

XVII. 

Cadmium  . 

XVIII. 

Aluminum 

XIX. 

Lead 

XX. 

Tin 

XXI. 

Electro-Metallui 

iGY    . 

Non-Me- 


PAGE 
9 

17 

34 
46 

74 

103 
116 
169 
187 
199 
202 
207 
217 
227 

235 
242 
244 

257 
260 
269 


CHAPTER  I. 

METALLURGY. 

THE  art  of  separating  metals  from  their  ores,  or 
from  simple  combinations  with  non-metallic  ele- 
ments, and  their  application  to  useful  purposes, 
may  be  regarded  as  a  separate  branch  of  chemical 
science.  It  is  essential  that  the  student,  before  com- 
mencing its  study,  should  acquire  a  good  preliminary 
knowledge  of  chemistry  and  mechanics. 

The  empirical  reduction  of  the  ores  of  metals  seems 
to  have  been  practiced  at  a  very  remote  period,  its 
origin  being  attributed  to  Tubal  Cain,  seventh  only 
in  descent  from  Adam.*  The  remains  of  numerous 
mines  have  been  discovered  on  the  southern  and 
eastern  borders  of  the  Ural  Mountains,  in  which  have 
been  found  hammers  and  chisels  of  copper,  and  other 
instruments  of  the  same  metal  of  which  the  uses  at 
present  are  unknown,  t  That  these  implements  were 
those  of  a  wandering  people  would  seem  to  be  evi- 
denced by  the  absence  of  any  traces  of  masonry  in 
the  neighborhood  ;  and  the  fact  that  no  iron  tools 
were  found  near  them  would  indicate  their  great 
antiquity. 

*In  the  fourth  chapter  of  Genesis  Tubal  Cain  is  spoken  of  as  an  "  in- 
structor of  every  artificer  in  brass  and  iron." 
t  Percey's  Metallurgy. 

2  9 


IO  DENTAL     METALLURGY. 

Gmelin  found  in  the  eastern  part  of  Siberia  the 
remains  of  nearly  one  thousand  smelting-furnaces, 
very  primitive  in  character,  surrounded  by  heaps 
of  scoria,  broken  pottery,  and  other  evidences  that 
metallurgical  operations  of  considerable  magnitude 
had  been  at  some  distant  period  carried  on  in  that 
locality.* 

The  alchemists  of  the  Middle  Ages  were  the  metal- 
lurgists par  excellence  of  that  period,  and  there  is 
evidence  that  they  were  acquainted  with  chemical 
processes  for  the  reduction  of  metals,  which  were 
brought  to  a  great  state  of  perfection,  thus  showing 
that  the  practical  part  of  metallurgy  was  far  in 
advance  of  the  theoretical,  t 

To  these  early  experimenters,  however,  must  be 
awarded  the  credit  of  great  industry.  They  knew 
nothing  of  the  metals  as  ultimate  bodies,  nor  of  the 

*  Percey's  Metallurgy. 

f  The  following  experiments,  from  manuscripts  discovered  by  M. 
Ferdinand  Hoefer,  will  serve  to  convey  an  adequate  idea  of  the  status  of 
metallurgy  from  the  third  and  fourth  centuries  down  to  a  comparatively 
recent  date  : 

"  Experiment  No.  i. — A  piece  of  red-hot  iron  is  placed  under  a  bell, 
which  rests  in  a  basin  lull  of  water.  The  water  diminishes  in  volume,  and 
a  candle,  being  introduced  into  the  bell,  sets  fire  at  once  to  the  gas  inside. 
Conclusion — water  changes  into  fire. 

"  Second  Experiment. — A  piece  of  lead,  or  any  other  metal  except  gold 
or  silver,  is  burned  in  contact  with  the  air.  It  immediately  loses  its  primi- 
tive properties,  and  is  transformed  into  a  powder  or  species  of  ashes  or 
lime.  The  ashes,  which  are  the  product  of  the  death  of  the  metal,  are 
again  taken  and  heated  in  a  crucible,  together  with  some  grains  of  wheat, 
and  the  metal  is  seen  rising  from  its  ashes  and  reassuming  its  original 
form  and  properties.  Conclusion — metals  are  destroyed  by  fire  and  re- 
vivified by  wheat  and  heat. 

"  Third  Experiment. — Argentiferous  lead  is  burned  in  cupels  composed 
of  ashes  of  pulverized  bones ;  the  lead  disappears,  and  at  the  end  of  the 
operation  there  remains  in  the  cupel  a  nugget  of  pure  silver.  Conclusion 
— lead  is  transformed  into  silver."  (It  is  probable  that  upon  this  and 
analogous  facts  was  founded  the  theory  of  the  transmutation  of  metals.) 


METALLURGY.  1 1 

particular  force  governing  their  union  with  the  non- 
metallic  elements,  and  finding  that  an  earthy  matter, 
such  as  an  ore  of  iron,  became  converted  by  fire  into 
a  metal,  they  naturally  believed  the  change  of  earth 
into  metals  to  be  possible,  and  in  the  search  for  gold, 
the  philosopher's  stone,  etc.,  they  really,  by  mere 
accident,  discovered  many  valuable  chemical  agents. 
In  this  way  sulfuric,  nitric,  and  hydrochloric  acids 
were  produced,  and  these,  made  to  act  upon  the  metals, 
in  turn  yielded  the  metalline  salts. 

Thus  it  will  be  seen  that  from  the  gradual  aggre- 
gation of  facts  resulting  from  the  pursuits  of  the 
alchemists  ultimately  sprang  an  exact  scienoe,  and 
toward  the  latter  part  of  the  sixteenth  century 
appeared  a  set  of  investigators  of  a  very  different 
order,  who,  instead  of  wasting  their  time  in  the  pur- 
suit of  such  fanciful  theories  as  that  of  transmutation, 
etc.,  devoted  themselves  to  the  unraveling  of  the 
principles  that  govern  the   composition    and   forma- 


"  Fourth  Experiment. — A  strong  acid  is  poured  on  copper;  the  metal 
is  acted  upon,  and  in  process  of  time  disappears;  or,  rather,  is  trans- 
formed into  a  green,  transparent  liquid.  Then  a  thin  plate  of  iron  is 
plunged  into  the  liquid,  and  the  copper  is  seen  to  reappear  in  its  ordi- 
nary aspect,  while  the  iron  in  its  turn  is  dissolved.  Conclusion — iron 
is  transformed  into  copper." 

Practically  the  fourth  experiment,  quoted  from  Jules  Andrieu's  paper 
on  "Alchemy,"  written  for  the  Encyclopaedia  Britannica,  is  electro- 
lysis, the  principle  by  which  a  compound  of  a  metal  with  a  non-metal 
is  decomposed  by  galvanic  electricity ;  but  the  transmutation  theory 
was  generally  accepted  as  accounting  for  the  phenomena  noticed  in 
this  experiment,  and  it  would  seem  that  at  least  some  of  the  savants 
of  the  period  were  sufficiently  shrewd  and  unscrupulous  to  turn  the 
process  to  profitable  account,  since  we  find  that  "  St.  Thomas  Aquinas, 
in  his  theological  writings,  forbids  the  sale  of  '  alchemist's  gold,'  and 
in  a  special  treatise  on  the  subject  unmasks  an  imposture  of  the  char- 
latans of  the  day,  who  pretend  to  make  silver  by  projecting  a  sublimate 
of  white  arsenic  on  copper.  " 


12  DENTAL   METALLURGY. 

tion  of  bodies  already  known.  Thus,  Paracelsus* 
was  the  first  to  distinguish  the  true  character  of 
some  of  the  well-known  salts,  such  as  alum  and  cop- 
peras, showing  that  they  contained  metals, — a  matter 
of  great  importance  at  that  day,  inasmuch  as  it 
eventually  led  to  the  discovery  that  many  of  the 
well-known  crystalline  salts  were  compounds  of  dis- 
similar elements,  as  a  metal  with  a  non-metallic  body, 
and  to  a  knowledge  of  the  particular  force  governing 
their  union  ;  and  finally  the  investigations  of  Beecher 
and  Stahl  of  Cronstadt,  Klaproth,  Wollaston,  Ber- 
zelius,  Wohler  and  Deville,  and  others,  have  dispelled 
many  illusions  and  rendered  accurate  the  present 
literature  of  the  subject.  During  the  latter  half  of  the 
'eighteenth  century  the  list  of  metals  was  augmented 
by  new  discoveries,  and  the  application  of  the  voltaic 
current  to  the  decomposition  of  the  alkalies  by  Sir 
Humphry  Davy  in  1807-8  added  a  dozen  or  more. 
The  employment  of  the  spectroscope  by  Kirchhoff 
and  Bunsen,  in  i860,  brought  to  light  so  many  new 
metals  that  the  total  number  now  exceeds  fifty. 

*  Paracelsus,  though  the  author  of  many  fanciful  doctrines,  seems  to 
have  been  the  first  to  offer  a  true  chemical  explanation  of  the  action  of 
mercury,  lead,  etc.,  upon  the  human  system. 


CHAPTER   II. 

THE  METALLIC  ELEMENTS. 

MODERN  CHEMISTRY  assumes  that  the  metals 
are  elementary  bodies,  yet  there  have  been  other 
theories  presented  regarding  their  ultimate 
character,  and  it  is  thought  by  some  that  "when 
man  shall  have  mastered  that  great  power  of  nature, 
electricity,  many  of  the  so-called  elements  will  be 
found  probably  to  be  compound  bodies."  Others 
have  entertained  the  theory  of  but  one  ultimate  ele- 
ment,* while  nearly  all  agree  that  as  we  advance  in 
knowledge  new  elements  will  be  brought  to  light. 

The  old  philosophers  applied  the  term  element  to 
imaginary  principles  of  matter,  such  as  fire,  water,  and 
air  ;  while  the  elements  of  the  alchemists  were  salt, 
sulfur,  and  mercury.  The  term  is  now  used  as  syn- 
onymous with  simple  body,  or  one  of  the  undecompos- 
able  constituents  of  any  kind  of  matter,  or  that  which 
cannot  be  divided  by  chemical  analysis. 

The  elements  known  at  present  number  sixty-seven, 
divided  into  the  metallic  and  non-metallic.  Of  the 
former  there  are  fifty-two,  as  follows  : 

*  Prof.  Graham's  Researches  with  Hydrogen,  in  1869. 

13 


14 


DENTAL   METALLURGY. 


• 

COMBINING 

NAMES. 

SYMBOLS. 

WEIGHTS. 

Aluminum. 

Al. 

27.4 

Antimony. 

Sb.  [Stibium) 

122 

Arsenic. 

As. 

75 

Barium. 

Ba. 

137 

Beryllium. 

Be.  (or  Glucinum.)         9.4 

Bismuth. 

Bi. 

210 

Cadmium. 

Cd. 

112 

Caesium. 

Cs. 

133 

Calcium. 

Ca. 

40 

Cerium. 

Ce. 

92 

Chromium. 

Cr. 

52.2 

Cobalt. 

Co. 

58.8 

Copper. 

Cu.  (Cuprum) 

63.4 

Davyum. 

Da. 

(?) 

Didymium. 

D. 

95 

Erbium 

E. 

168.9 

Gallium. 

Ga. 

68 

Gold. 

Au.  (Atirum) 

197 

Indium. 

In. 

H3-4 

Iridium. 

Ir. 

198 

Iron. 

Fe.  (Ferrum) 

56 

Lanthanum . 

La. 

93-6 

Lead. 

Pb.  (Plumbum) 

207 

Lithium. 

Li. 

7 

Magnesium. 

Mg. 

24 

Manganese. 

Mn. 

55 

Mercury. 

Hg.  (Hydrargyrum)  200 

Molybdenum. 

Mo. 

96 

Nickel. 

Ni. 

58.8 

Niobium. 

Nb. 

94 

Osmium. 

Os. 

199.2 

Palladium. 

Pd. 

106.6 

Platinum. 

Pt. 

197.4 

Potassium. 

K.  (Kalium) 

39- 1 

Rhodium. 

Rh. 

104.4 

Rubidium. 

Rb. 

85.4 

Ruthenium. 

Ru. 

104.4 

THE    METALLIC    ELEMENTS, 


15 


COMBINING 

NAMES. 

SYMBOLS. 

WEIGHTS. 

Silver. 

Ag.  {Argentum) 

IOS 

Sodium. 

Xa.  {Natrium) 

23 

Strontium. 

Sr. 

87.6 

Tantalum. 

Ta. 

lS2 

Terbium. 

Ter. 

148.5 

Thallium. 

Tl. 

204 

Thorium. 

Th. 

235 

Tin. 

Sn.  ( Statmum) 

Il8 

Titanium. 

Ti. 

50 

Tungsten. 

W.  (  Wolframium 

0    184 

Uranium. 

U. 

240 

Vanadium. 

V. 

51-2 

Yttrium. 

Y. 

92 

Zinc. 

Zn. 

65.2 

Zirconium. 

Zr. 

89.6 

Of  these,  only  about  fifteen  are  employed  in  true 
metallic  condition.     These  are  : 

Antimony,  Lead, 

Aluminum,  Magnesium, 

Bismuth,  Mercury, 

Copper,  Nickel, 

Gold,  Platinum, 

Iridium,  Silver, 

Iron,  Tin, 

Zinc. 

Twelve  are  more  or  less  extensively  used  in  medi- 
cine, and  in  the  arts  as  coloring  pigments  and  for 
alloying  purposes.     These  are  : 


Arsenic, 

Barium, 

Cadmium, 

Calcium, 

Chromium, 

Cobalt, 


Lithium, 

Manganese, 

Potassium, 

Sodium, 

Titanium, 

Uranium. 


l6  DENTAL     METALLURGY. 

The  remaining  twenty-five  are  as  yet  of  little  or 
no  practical  use  in  the  metallic  state. 

Seven  of  the  metals  play  a  more  or  less  important 
part  in  the  maintenance  of  animal  and  vegetable  life. 
These  are  : 

Aluminum,  Manganese, 

Calcium,  Potassium, 

Iron,  Sodium. 
Magnesium, 

The  metallic  elements  are  divided  by  metallurgists 
into  two  classes, — the  noble  and  base  metals.  The  first 
are  those  which  are  capable  of  being  separated  from 
combinations  with  oxygen  by  merely  heating  to  red- 
ness. The  base  metals  are  those  whose  compounds 
with  oxygen  are  not  decomposable  by  heat  alone. 

The  noble  metals  are  ten  in  number,  as  follows  : 


Mercury. 

Hg. 

200 

Silver. 

Ag. 

108 

Gold. 

Au. 

197 

Platinum. 

Pt. 

197.4 

Palladium. 

Pd. 

106.6 

Rhodium. 

Rh. 

104.4 

Ruthenium. 

Ru. 

104.4 

Osmium. 

Os. 

199.2 

Iridium. 

Ir. 

198 

Davyum. 

Da. 

(?) 

The  base  metals  are  further  subdivided  according 
to  their  affinity  for  oxygen  and  other  chemical  prop- 
erties. 


CHAPTER     III. 

PROPERTIES  OF  THE  METALS. 

A  METAL  may  be  defined  as  an  elementary  sub- 
stance, usually  solid  at  ordinary  temperatures,* 
insoluble  in  water,  fusible  by  heat,  and  possessing 
a  peculiar  luster,  commonly  spoken  of  as  a  "  metallic 
luster,"  an  expression  sometimes  used  in  describing 
the  appearance  of  substances  which  present  a  similar 
condition  of  surface.  To  these  qualities  must  be 
added  those  of  conducting  heat  and  electricity,  which 
the  metals  possess  to  the  greatest  extent,  and  the 
power  of  the  metals  of  replacing  hydrogen  in  chemi- 
cal reactions  ;  as,  when  zinc  is  placed  in  contact  with 
hydrochloric  acid,  it  displaces  the  hydrogen  and  unites 
with  the  chlorin  to  form  zincichlorid  (chlorid  of  zinc), 
thus  : 

Zn+2HCl  =  ZnCl24-H2  liberated. 

Another  characteristic  of  the  metals  is  their  basic 
properties  when  united  with  oxygen. 

Arsenic  and  tellurium  are  by  some  regarded  as 
intermediate  links  between  the  metallic  and  non- 
metallic  bodies.  Watts,  in  his  "  Dictionary  of  Chem- 
istry," says  of  tellurium  that  "this  element,  though 
decidedly  metallic,  must  be  classed  as  a  member  of 

*  Mercury  is  an  exception,  being  fluid  at  the  ordinary  temperature.     It 
freezes  at — 400  F. 

17 


1 8  DENTAL     METALLURGY. 

the  sulfur  family,"  probably  in  consequence  of  its 
poor  conducting  qualities  and  the  acid  character  of  its 
oxids. 

Bloxam  does  not  regard  arsenic  as  a  metal,  and 
states  that,  though  "some  authorities  class  it  as  such 
on  account  of  its  metallic  luster  and  property  of  con- 
ducting electricity,  yet  it  is  lacking  in  the  quality  of 
forming  a  base  with  oxygen,  a  property  common  to 
all  the  true  metals  ;"  and  asserts  that  "the  chemical 
character  and  composition  of  its  compounds  connect 
it  in  the  closest  manner  with  the  phosphorus  group." 

On  the  other  hand,  we  find  some  of  the  non-metallic 
bodies  possessing  the  chemical  but  not  the  physical 
properties  of  the  metals.  Thus,  the  real  nature  of 
hydrogen  has  long  been  an  interesting  point  of  dis- 
cussion among  chemists,  some  supposing  it  to  be  a 
metal  in  a  gaseous  form.  Dumas  and  others  prophe- 
sied that  ' '  if  ever  the  means  of  liquefying  hydrogen 
is  found,  it  will  present  the  appearance  of  quicksilver, ' ' 
and  their  grounds  for  this  belief  are  its  uniformly  basic 
properties.  Others  contend  that  it  is  a  neutral  sub- 
stance, possessing  both  the  basic  properties  of  a  metal 
and  the  chlorous  qualities  of  a  gas. 

In  1869  it  was  announced  that  Professor  Graham,  an 
eminent  English  chemist,  had  discovered  the  metallic 
hydrogen.  ' '  This  new  metal,  baptized  '  hydroge- 
nium,'  was  white,  magnetic,  of  a  specific  gravity 
about  2,  and  appeared  to  have  some  analogy  to  mag- 
nesium." This  discovery  excited  much  speculation. 
Upon  verification,  however,  the  new  metal  was  found 
to  be  a  compound  of  palladium  and  hydrogen,  in 
which  the  former  had  absorbed  700  or  800  times  its 
bulk  of  the  latter. 


PROPERTIES  OF   THE   METALS.  iy 

Again,  the  existence  of  a  hypothetic  compound 
metal  called  ammonium,  and  having  the  constitution 
NH4,  has  been  assumed  as  the  only  method  of  ex- 
plaining the  perfect  analogy  that  exists  between  the 
salts  of  ammonium  and  those  of  some  of  the  metals, 
actual  experiments  having  already  strengthened  this 
theory,  at  first  founded  only  on  analogy.* 

The  metals  are  all  quite  opaque,  with  the  single 
exception  of  gold,  which,  however,  is  only  transpa- 
rent in  leaves  of  a  highly  attenuated  condition,  when 
it  transmits  green  light,  f 

The  Color  of  the  metals  ranges  from  the  pure  white 
of  silver  to  the  bluish  hue  of  lead.  Between  these 
two  the  major  part  of  the  others  may  be  found. 
About  five  run  from  light  yellow  to  deep  red.  These 
are,  barium  and  strontium,  pale  yellow ;  calcium, 
somewhat  deeper  in  color ;  gold,  when  pure,  of  a 
rich  yellow  ;  and  copper,  the  only  red  metal.  It  was 
at  one  time  supposed  that  the  mineral  titanium,  well 
known  to  dentists  as  a  dark  red  (copper-colored), 
crystalline  substance,  used  in  a  finely  divided  state 
as  a  coloring  pigment  in  the  manufacture  of  por- 
celain teeth,  was  a  metal.  It  was  so  pronounced  by 
Wollaston.  Wohler  and  Deville,  however,  demon- 
strated that  the  red  mineral  is  an  oxid,  and  they 
verified  their  statement  by  producing  the  metal  it- 
self, which  is  of  a  steel-gray  color.  The  color  of  the 
metals  is  modified  by  alloying.  J 

Luster. — This  characteristic  of  the  metals  is  prob- 

*  E.  Miller,  Treatise  on  Chemistry. 

f  It  is  by  some  believed  that  the  absence  of  transparency  in  the  other 
metals  may  only  depend  upon  our  inability  to  obtain  them  in  a  sufficiently 
attenuated  condition. 

\  See  chapter  on  "Alloys." 


20  DENTAL     METALLURGY. 

ably  the  result  of  perfect  opacity.,  by  which  the  rays 
of  light  are  reflected  from  the  surface. 

Odor  and  Taste  are  possessed  by  some  few  of  the 
metals.  The  greater  number,  however,  are  destitute 
of  these  qualities.  Iron,  copper,  and  zinc,  when 
heated,  evolve  peculiar  odors,  and  one  means  of  de- 
tection of  arsenic  is  the  odor  of  garlic  observed  when 
that  metal  is  exposed  to  an  elevated  temperature. 
Odor  and  taste  may  depend  upon  voltaic  action.  The 
former  may  be  noticed  in  a  marked  degree  when 
holding  in  the  hand  a  mass  of  an  alloy  composed  of 
gold,  platinum,  tin,  and  silver  prepared  for  use  as 
amalgam.  The  moisture  of  the  hand,  aided  by  its 
heightened  temperature,  seems  to  promote  the  elec- 
trical action. 

Fusibility. — All  metals  admit  of  being  reduced  to  a 
liquid  state  by  the  application  of  heat,  but  the  tem- 
perature at  which  they  melt  differs  widely.  Thus, 
mercury  retains  its  liquid  form  to  390  F.  below  zero, 
and  is  always  fluid  at  ordinary  temperatures.  Po- 
tassium and  sodium  fuse  below  the  boiling-point  of 
water  ;  tin,  lead,  and  antimony  below  redness.  Gold, 
silver,  and  copper  require  bright  redness.  Iron, 
nickel,  and  cobalt  fuse  at  white  heat,  while  platinum, 
iridium,  rhodium,  titanium,  etc.,  become  fluid  only 
when  exposed  to  a  powerful  voltaic  current  or  the 
flame  of  the  oxyhydrogen  blow-pipe. 


PROPERTIES    OF   THE    METALS. 


21 


Table  of  Fusing-points  of  the  Principal  Metals. 


o5 
u 

'55 


0)  rt 


en  o    • 

<y    O    p 

-13  "S  ^ 

*>2i  o 


r 


&2  g^ 


^  CWD  CD 

c  o  a 
o  *•* 


'•a  a-j 


CD   >>  > 

tin 


Mercury 

Rubidium 

Potassium 

Sodium 

Lithium 

Tin 

Cadmium 

Bismuth 

Thorium 

Lead     . 

Tellurium,  rather  less 

Arsenic,  unknown. 

Zinc 

Antimony,  just  below 


FAHR. 

-39° 
+101.3 

144-5 
207.7 

356 
442 

442 

497 
56i 
617 
fusible  than  lead 


CENT. 

— 39-44c 
+38.5 
62.5 

97.6 
180 
227.8 
227.8 
258 
294 

325 


Silver  . 
Copper 
Gold    . 

Cast  Iron 
Pure  Iron 
Cobalt 
Manganese 
Palladium 

Molybdenum 

Uranium 

Tungsten 

Chromium 

Titanium 

Cerium 

Osmium 

Iridium 

Rhodium 

Platinum 

Tantalum 


redness. 


773 

FAHR. 

1873 
I996 
20I& 
2786 


412 

CENT. 
IO23 
I09I 
1 102 

I530 


22 


DENTAL     METALLURGY, 


Capacity  for  Heat. — The  metals,  in  common  with 
other  bodies,  have  their  specific  heat.  This  consists 
in  the  amount  of  heat  required  to  raise  equal  weights 
of  different  metals  from  the  same  to  another  given 
temperature.  Thus,  if  we  express  by  i  the  quantity 
of  heat  necessary  to  raise  a  weight  of  water  from 
o°  C.  to  i°  C,  that  which  must  be  supplied  to  elevate 
the  same  weight  of  the  following  metals  to  that  tem- 
perature would  be  as  follows  :  * 


Mercury    . 

0.03332 

Gold  . 

0.03244 

Iron    . 

0.1 138 

Nickel 

• 

0.1086 

Cobalt 

0. 1070 

Zinc    . 

0.0956 

Copper 

0.0952 

Palladium  , 

0.0593 

Silver  . 

0.0570 

Cadmium  . 

0.0567 

Tin       .       . 

0.0562 

Antimony  . 

0.0508 

Lead  . 

0.0314 

Platinum    . 

0.0311 

Bismuth     . 

0.0308 

Now,  if  we  should  take  equal  bulks  of  these  metals 
and  expose  them  for  the  same  length  of  time  to 
exactly  the  same  heat,  and  then  place  them  simul- 
taneously upon  a  cake  of  wax,  we  would  observe 
those  of  the  above  table  with  the  highest  figures,  such 
as  iron,  instantly  melting  their  way  through  the  wax, 
while  those  of  the  lowest  capacity  for  heat,  such  as 
bismuth,  would  remain  on  the  surface. 


*  Phillips's  Metallurgy,  p.  13. 


PROPERTIES    OF   THE   METALS.  23 

Expansion  by  Heat. — Metals  expand  when  heated, 
but  this  property  is  not  uniform,  some  possessing  it 
to  a  greater  or  less  extent  than  others.  Within  cer- 
tain limits  of  temperature  this  takes  place  propor- 
tionately to  the  amount  of  heat  to  which  they  are 
exposed.  Zinc  possesses  a  rather  high  degree  of 
expansibility,  and  is  consequently  useful  for  the  pur- 
pose of  making  dies  for  swaging  metal  plates  for 
artificial  dentures.  By  many  dentists  it  was  for- 
merly thought  that  a  metal,  to  be  well  suited  for  this 
purpose,  should  be  entirely  destitute  of  this  property, 
so  that  after  casting  the  die  should  not,  in  returning 
to  its  former  condition  in  cooling,  be  smaller  than  the 
plaster  model,  the  objector  se  being  to  have  the  plate 
fit  the  plaster  cast  perfectly  ;  whereas,  the  real  pur- 
pose should  be  to  make  the  plate  fit  the  mouth  closely, 
the  plaster  model  being  only  a  means  to  that  end. 
Plaster  expands  in  setting.  From  the  impression 
to  the  model  two  expansions  are  gone  through  before 
the  fac-simile  of  the  mouth  in  plaster  is  obtained  ; 
hence,  a  plate  made  to  fit  such  a  model  perfectly  must 
necessarily  be  somewhat  larger  than  the  mouth, — a 
condition  unfavorable  to  atmospheric  adhesion.  On 
the  other  hand,  a  plate  made  to  fit  the  zinc  will 
not  be  found  too  small  for  the  mouth,  but  will,  pro- 
vided the  impression  is  a  good  one  and  represents 
perfectly  the  conformation  of  the  mouth,  afford  a 
very  close-fitting  plate.  Even  better  results  might 
be  expected  where  the  plate  is  somewhat  smaller 
than  the  mouth,  because  such  a  condition  would,  in 
entire  upper  dentures,  throw  any  undue  pressure  upon 
the  alveolar  ridge,  while  that  portion  of  the  plate 
covering  the  palatine  arch  would  barely  be  in  contact 


24  DENTAL     METALLURGY. 

with  the  tissues  ;  the  pressure  along  the  ridge  would 
quickly  promote  absorption  of  the  remains  of  the 
alveoli,  and  a  uniform  adaptation  of  the  plate  to  the 
mouth  would  soon  follow.  On  the  contrary,  if  the 
plate  be  made  to  fit  the  plaster  cast,  and  is  a  trifle 
larger  than  the  mouth,  the  pressure  will  be  thrown 
upon  the  palatine  arch  at  the  back  edge  of  the  plate, 
at  a  region  not  likely  to  change  by  absorption,  as  is 
the  case  with  the  alveolar  ridge,  and  hence  the  margin 
of  the  plate  will  imbed  itself  in  the  tissues  and  cause 
much  discomfort  and  impair  the  usefulness  of  the 
denture. 

Much  time  and  thought  have  been  expended  in  the 
effort  to  discover  some  alloy  which,  in  connection 
with  the  properties  of  hardness  and  fusibility,  shall 
possess  that  of  non-expansibility  when  heated.  Har- 
ris's "  Principles  and  Practice  of  Dentistry"  gives  no 
less  than  nine  different  formulae.  The  author  is  satis- 
fied that  the  property  of  expansibility  in  zinc  as  used 
in  the  dental  laboratory  constitutes  one  of  its  most 
valuable  qualities,  as  it  gives  us  the  means  of  com- 
pensating for  the  yielding  of  the  tissues  and  the 
absorption  along  the  ridge  which  nearly  always  fol- 
lows the  first  insertion  of  an  artificial  denture. 

Table  of  Expansion  of  Metals  for  each  degree  from  o°  C.  to  ioo°  C* 


Gold     ....  0.00155155 

Silver  ....  0.00190868 

Platinum  .     .     .  0.00099180 

Palladium     .     .  0.00100000 

Copper     .     .     .  0.00171733 

Iron     ....  0.00123504 


Lead     ....  0.00284836 

Tin 0.00193765 

Zinc  (cast)    .     .  0.00294167 

"    (ham'r'd) .  0.00310833 

Bismuth  .     .      .  0.00139167 

Antimony     .     .  0.00108333 


*  Phillips's  Metallurgy. 


PROPERTIES    OF    THE    METALS. 


25 


Power  of  Conducting  Heat. — The  metals  are  the 
best  conductors  of  heat  among  the  solid  bodies. 
The  quality  of  transmitting  heat  is  possessed  by 
them  in  variable  degrees.  The  following  table  shows 
the  relative  approximate  ratio  of  conductivity  of  heat 
of  each  of  the  metals  commonly  used  in  the  mechanic 
arts  : 

Silver 


100 


J11VC1                 ...... 

Copper    

1UU 

73-6 

Gold         ...                 .         . 

53-2 

Brass* 

23.6 

Tin 

14.5 

Iron          .... 

11. 9 

Steel         ... 

11. 6 

Lead 

•         •          8.5 

Platinum           .... 

.         .          8.4 

German  Silver 

6.3 

Rose  Fusible  Metal 

2.8 

Bismuth 

..      •          1.8 

Power  of  Conducting  Electricity. — Metals  conduct 
electricity  nearly  in  the  ratio  of  their  capacity 
of  transmitting  heat.  Davy,  Becquerel,  and  Dr. 
Matthiesen  have,  at  different  times,  more  or  less 
extensively  experimented  upon  this  characteristic 
quality  of  the  metals.  Among  the  results  of  Dr. 
Matthiesen' s  investigations  are  the  facts  that  debas- 
ing a  metal  or  alloying  it  greatly  diminishes  its  con- 
ducting power,  and  that  elevation  of  temperature 
has  the  same  effect,  and  that  between  320  and  2120  F. 
(or  o°  and  ioo°  C),  great  diminution  takes  place, — 
not  uniformly,  however,  as  some  lose  it  more  in  pro- 
portion than  others.f 

*Zinc  is  probably  between  brass  and  tin. 
t  Makins's  Metallurgy-,  p.  17. 


26  DENTAL     METALLURGY. 

A  rough  means  of  determining  the  relative  con- 
ducting power  of  metals  consists  in  connecting  the 
poles  of  a  voltaic  battery  by  a  wire  through  which 
the  current  will  pass  freely.  Now,  if  the  wire  be  too 
small  for  the  transmission  of  the  electricity  supplied 
to  it,  the  obstruction  will  be  manifested  by  the  wire 
becoming  red-hot.* 

Hence  the  relative  capacity  of  metals  for  this  pur- 
pose may  be  observed  by  employing  equal  battery- 
power  upon  wires  of  the  same  diameter  of  different 
metals,  and  noting  the  length  of  the  portion  of  each 
which  can  thus  be  heated. 

Conversely,  the  same  means  may  be  employed  to 
indicate  the  quantity  of  electricity,  or  the  capacity 
of  the  battery  itself.  In  this  case  the  wire  is  made 
to  demonstrate  the  power  of  the  battery  by  the  length 
of  wire  which  the  battery  is  capable  of  rendering 
incandescent. 

A  good  demonstration  of  the  relative  conducting 
power  of  different  metals  may  be  made  by  construct- 
ing a  chain  of  alternate  links  of  platinum  and  silver 
wire.  This  will  show,  while  the  current  of  electricity 
is  passing,  red-hot  platinum  links  alternating  with 
cool  silver  ones.  Platinum,  being  much  the  inferior 
conductor,  offers  such  an  impediment  to  the  passage 
of  the  current  that  great  elevation  of  temperature 
results,  while  the  silver,  being  a  good  conductor, 
offers  no  check  to  the  free  passage  of  the  electricity. 

The    power  of  metals  of  conducting    electricity  is 


*  This  was  fully  shown  in  some  of  the  early  experiments  with  the 
electro-magnetic  mallet,  in  which  the  wire  was  too  small  for  the  accom- 
panying battery  power.  They  worked  very  well  for  <t  few  minutes,  when 
they  became  hot  and  ceased  working. 


PROPERTIES    OF   THE    METALS. 


27 


shown  in  the  following  table  from  Matthiesen  (Phil. 
Trans.  1863)  : 

Silver 100      at  320  F. 


Copper 

Gold 

Zinc 

Iron 

Tin 

Lead 

Antimony 

Bismuth 


100 

99-95  " 

77.96  " 

29.02  " 

16.81  " 

12.36" 

8.32" 

4.62  " 

1.24" 


Malleability,  Ductility,  and  Tenacity. — The  qualities 
of  malleability,  ductility,  and  tenacity  differ  widely 
in  the  metals.  The  term  Malleability,  when  applied 
to  such  a  metal  as  gold,  signifies  that  by  hammering 
or  rolling  its  surface  may  be  extended  in  all  directions, 
and  that  it  is  capable  of  being  thus  reduced  to  very 
thin  leaves  or  sheets  without  fracture  of  its  continuity 
at  the  edges  during  the  process  of  attenuation  ;  when 
applied  to  other  metals,  the  term  should  be  understood 
as  expressing  this  quality  relatively.  Gold  is  the  most 
malleable  of  the  metals,  and  is  capable  of  being  made 
into  leaves  of  -g-owcro"  °f  an  ^ncn  m  thickness,  each 
grain  of  which  will  cover  a  surface  of  54  sqr.  inches. 

In  the  following  list,  by  Regnault,*  the  metals  are 
arranged  in  the  order  of  their  malleability  : 


1.  Gold. 

2.  Silver. 

3.  Tin. 

4.  Copper. 

5.  Cadmium. 

6.  Platinu 

7.  Lead. 

8.  Zinc. 

9.  Iron. 
10.  Nickel. 

m.         11.  Palladium. 

12.  Potassium. 

13.  Sodium. 

14.  Mercury  (frozen). 

Ductility  signifies    that 

property  which    renders  a 

*  Phillips's  Metallurgy,  p.  412. 


28  DENTAL    METALLURGY. 

metal  capable  of  being  drawn  into  rods  or  wires, 
usually  accomplished  by  passing  an  elongated  piece 
of  metal  through  a  series  of  gradually  diminishing 
holes  in  a  steel  draw-plate  ;  the  granular  particles  of 
the  metal  are  thus  extended  into  fibers.  One  grain  of 
gold  has  been  drawn  into  a  wire  550  feet  long.  To 
accomplish  this  result  a  compound  wire  is  made,  of 
gold  covered  with  silver,  the  tenacity  of  the  latter 
being  taken  advantage  of  to  enable  the  gold  to  be 
carried  through  the  successive  holes  of  the  draw- 
plate,  until  the  greatest  possible  attenuation  is 
reached  ;  after  which  it  is  immersed  in  nitric  acid, 
which  dissolves  the  silver,  leaving  a  gold  wire  -5-0V0 
of  an  inch  in  diameter. 

In  the  following  table  the  metals  are  arranged 
according  to  their  ductility  : 

1.  Gold.  5.  Copper.  9.  Nickel. 

2.  Silver.  6.  Zinc.  10.  Palladium. 

3.  Platinum.  7.  Tin.  11.  Cadmium. 

4.  Iron.  8.  Lead. 

Tenacity  is  the  power  possessed  by  metals  of  sus- 
taining weight,  or  of  resisting  rupture,  when  a  bar 
or  rod  is  exposed  to  tension.  As  the  fitness  of  metals 
for  certain  purposes  in  the  industrial  arts  depends 
largely  upon  this  property,  it  is  of  the  utmost  im- 
portance to  know  the  relative  tenacity,  not  only 
of  the  different  metals,  but  of  different  alloys.  This 
is  usually  ascertained  by  preparing  wires  of  exactly 
equal  diameters.  These  are  suspended  by  one  end 
from  a  fixed  bar,  and  to  the  other  extremity  weights 
are  gradually  and  carefully  added  until  the  wire 
breaks.     The  weight  which  causes  the  fracture  rep- 


PROPERTIES    OF   THE    METALS.  29 

resents,  when  compared  with  other  wires  similarly- 
treated,  the  relative  tenacity  of  the  metal. 

Elevation  of  temperature,  even  within  rather  cir- 
cumscribed limits,  affects  the  tenacity  of  metals  to  a 
marked  degree,  generally  diminishing  it.  There  are 
some  exceptions,  such  as  iron  and  steel.  On  the  other 
hand,  malleability  and  ductility  are  only  developed 
in  some  of  the  metals  by  an  elevated  temperature. 
Thus,  it  was  found  that  zinc,  which  had  previously 
been  of  no  use  in  an  unalloyed  state,  was  rendered 
perfectly  malleable  and  capable  of  being  rolled  into 
very  thin  sheets  merely  by  heating  to  between  2480 
and  3020  F.  (=120°  and  I50°C),  and  it  has  conse- 
quently come  very  largely  into  use.  If  carried  much 
beyond  this  point,  however,  say  to  4000  F.  (=205° 
C),  it  becomes  very  brittle,  and  may  even  be  reduced 
to  powder  in  an  iron  mortar.  A  rather  unsatisfactory 
demonstration  of  this  fact  sometimes  occurs  in  the 
fracture  incident  to  the  falling  upon  the  hearth  or 
floor  of  a  hot  zinc  die,  carelessly  removed  from  the 
molding  sand  in  the  laboratory,  its  brittleness  being 
so  extreme  at  5000  F.  that  it  might  be  broken  into 
a  number  of  pieces. 

Magnesium,  aluminum,  and  some  other  metals, 
which  at  ordinary  temperatures  are  nearly  destitute 
of  ductility,  have  that  quality  greatly  increased  by 
heating,  and  are  then  readily  drawn  into  wire. 

In  alloys  these  qualities  are  diminished  by  heating. 
Thus,  the  great  tenacity  and  ductility  of  brass  are 
entirely  destroyed  by  simply  heating  to  dull  redness. 
Again,  while  it  is  claimed  that  in  pure  gold  tenacity 
is  increased  by  heating,  it  is  quite  certain  that  18- 
carat  gold  is  rendered  brittle  at  red  heat. 


3Q 


DENTAL    METALLURGY. 


The  following  table*  gives  the  results  of  experi- 
ments on  the  tensile  strength  of  a  few  of  the  metals  at 
temperatures  between  150  and  200  C. 


NAME  OF  METAL 

For  wire  of  i  Sq.  MM.  Section, 
Weight  (in  Kilos)  causing 

Permanent  Elonga- 
tion of  1-20,000. 

Breakage. 

Gold,  drawn 

"      annealed  . 
Silver,  drawn 

"      annealed  . 
Platinum,  drawn 

' '         annealed 
Copper,  drawn   . 

annealed 
Iron,  drawn 

"     annealed   . 
Palladium,  drawn 
annealed 

13-5 
3-o 

"■3 

2.6 
26 

14 

12 
under  3 

32 
under  5 

18 
under   5 

27 
IO 
29 
16 

37 
23 
40 

30 
61 

47 
37 
27 

The   following   table   shows   the   order  of  relative 
capacity  of  the  metals  for  sustaining  weight : 


1.  Iron. 

4.  Silver. 

7.  Tin. 

2.  Copper. 

5.  Gold. 

8.  Lead 

3.  Platinum. 

6.  Zinc. 

It  has  been  observed  that  students  and  others  very 
often  fail  at  first  to  appreciate  the  difference  between 
these  properties,  and  they  not  infrequently  fall  into 
the  mistaken  idea  that  the  three  qualities  of  mal- 
leability, ductility,  and  tenacity  are  possessed  to  an 
equal  extent  by  each  metal.  If,  however,  we  take 
gold,  for  example,   the  most  perfectly  malleable  and 


*  Annales  de  Chimie  et  de  Physique  (iii),  vol.  xii,  Wertheim. 


PROPERTIES    OF   THE   METALS.  3 1 

ductile  of  the  metals,  we  shall  find  that  in  tenacity 
it  ranks  considerably  below  some  of  the  others,  and 
the  greatest  care  is  necessary  in  drawing  a  piece  of 
pure  gold  into  even  a  moderately  fine  wire,  and  be- 
yond a  certain  limit,  past  which  platinum  or  copper 
may  be  carried  with  safety,  gold  would  not  possess 
sufficient  tenacity  to  overcome  the  resistance  to  which 
it  would  be  exposed  in  passing  through  the  smaller 
holes  of  the  draw-plate,  and  fracture  would  result. 

Iron,  on  the  other  hand,  which  exceeds  all  the  other 
metals  in  tenacity,  is  in  malleability  inferior  to  gold, 
silver,  copper,  platinum,  lead,  zinc,  tin,  and  cadmium. 

Crystalline  metals,  such  as  bismuth,  antimony,  and 
arsenic,  do  not  possess  these  properties.  They  are 
easily  broken  by  even  slight  blows  of  a  hammer,  and 
the  two  latter  may  be  reduced  to  powder  in  a  mortar. 

It  is  stated  that  brass  drawn  into  wire  will  often, 
after  a  time,  become  crystalline  in  texture  and  brittle 
by  slow  change  of  molecular  arrangement.* 

Crystallization. — It  is  stated  that  under  favorable 
circumstances  all  the  metals  will  assume  a  crystalline 
form.  It  is  known  that  some  of  them,  as  gold,  silver, 
etc.,  are  found  native  as  cubes  or  octahedra,  or  in 
slight  modifications  of  these  forms  ;  and  metals  in  a 
crystalline  form  may  be  obtained  by  electrolysis. 
For  example,  silver  may  be  obtained  in  the  form  of 
crystals  nearly  pure  by  introducing  strips  of  copper 
into  a  solution  of  argentic  nitrate.  A  piece  of  zinc 
introduced  into  a  solution  of  plumbic  nitrate  will 
precipitate  the  lead  in  the  form  of  feathery  crystals. 
Gold  may  also  be  deposited  in  this  form  from  solution 
by  the  introduction  of  a  stick  of  phosphorus.     Nearly 

*  Makins  s  Metallurgy,  p.  ro. 


32  DENTAL    METALLURGY. 

all  the  metals  yield  crystals  when  deposited  from 
their  solutions  by  electric  currents  of  feeble  intensity. 
The  beautiful  preparation  known  as  Watts' s  Crystal 
Gold  is  formed  in  this  way.  Gold  so  prepared  is 
generally  in  a  high  state  of  purity. 

Elasticity  and  Sonorousness  may  be  conferred  upon 
the  metals  by  alloying.  Thus,  iron  does  not  possess 
these  qualities  until  combined  with  the  proper  pro- 
portions of  carbon,  when  by  subsequent  tempering 
the  highest  degree  of  elasticity  is  developed,  and 
pieces  of  steel  of  different  lengths,  as  arranged  in  the 
dulcimer,  when  struck  by  a  small  wooden  hammer, 
are  capable  of  giving  off  the  most  musical  sounds. 

Again,  a  very  great  amount  of  elasticity  is  obtained 
by  the  admixture  of  copper  and  zinc  in  the  form  of 
brass,  from  which  a  spiral  spring  may  be  made,  equal 
to  that  from  any  other  alloy. 

It  is  curious  to  observe  how  this  quality  may  be 
developed  by  the  admixture  of  two  metals,  each  of 
which,  examined  separately,  is  soft  and  destitute  of 
anything  like  springiness.  Thus,  gold  and  platinum, 
both  soft  metals,  when  combined  in  the  proportions 
of,  say,  i  grain  of  the  latter  to  i  dwt.  of  the  former, 
of  20-carat  fineness,  will  afford  a  decidedly  elastic 
alloy,  suitable  for  clasps  for  artificial  dentures. 

A  tolerably  elastic  alloy  may  be  formed  by  combin- 
ing platinum  with  a  small  amount  of  iridium.  This 
alloy  is  frequently  employed  in  the  construction  of 
artificial  dentures.* 

Sonorousness  is  obtained  to  the  greatest  extent  in 
alloys  of  copper  and  tin,  known  as  bell-metal. 

Volatility. — All  metals  are  probably  more  or  less 

*See  chapter  on  "Platinum  and  its  Alloys." 


PROPERTIES   OF   THE   METALS.  33 

volatile,  although  only  a  certain  number  admit  of 
being-  converted  with  any  degree  of  facility  into  a 
state  of  vapor,  even  at  the  highest  temperatures. 
Some  of  the  conspicuously  volatile  metals  are  zinc, 
cadmium,  mercury,  arsenic,  tellurium,  potassium,  and 
sodium  ;  while  a  few  others  have  the  property  of 
communicating  characteristic  colors  to  flame,  and  are 
probably  volatile  to  a  limited  extent. 

Metals  are  sometimes  characterized  as  "fixed,"  as 
gold,  copper,  nickel,  etc.,  and  "volatile"  (during 
fusion),  as  cadmium,  zinc,  etc.  Arsenic  may  unques- 
tionably be  regarded  as  belonging  to  the  latter  group, 
passing  as  it  does  without  fusion  from  the  solid  to  the 
gaseous  state. 

Gold  has  been  known  to  volatilize  under  certain 
conditions.  Makins  states  that  it  is  doubtful  whether 
it  is  at  all  volatile  per  se,  but  if  alloyed  with  copper 
it  has  been  shown  by  Napier  to  be  considerably  vol- 
atilized, so  that  quantities  amounting  to  4^  grains 
could  be  collected  during  the  pouring  of  30  pounds' 
weight  from  a  crucible.  According  to  Makins,  gold 
has  been  known  to  volatilize  when  mixed  with  silver 
and  lead  and  cupelled  together,  he  having  collected 
considerable  quantities  of  each  metal  from  the  chim- 
ney of  an  assay  furnace  after  only  a  few  weeks'  use. 

Agents  which  may  Volatilize  a  Metal.  — Concentration 
of  solar  rays  in  the  focus  of  a  lens  ;  the  voltaic  cur- 
rent ;  the  oxyhydrogen  blow-pipe  flame. 

M.  Despretz  employed  the  three  in  conjunction,  by 
which  means  he  volatilized  magnesium,  and  with 
a  powerful  Bunsen  battery  alone  he  reduced  carbon 
by  volatilization  to  the  state  of  a  black  powder.* 

*  Percey's  Metallurgy. 


CHAPTER   IV. 

ALLOYS. 

MOST  of  the  metals  are  capable  of  uniting  with 
one    another,  forming    a    class  of  compounds 
termed  alloys,  in  which  may  be  observed  to  a 
greater  or  less  extent  the  properties  of  the  several 
constituents  entering  into  the  union. 

From  a  purely  scientific  point  of  view,  the  study 
of  the  alloys  is  an  interesting  one,  as  they  are  not 
only  mixtures  of  metals  possessing  certain  distinct 
qualities,  but  in  reality  are  true  chemical  compounds. 
In  the  appearance  which  often  accompanies  the  union 
of  the  metals,  and  in  the  properties  of  the  resulting 
alloys,  we  may  frequently  observe  the  phenomena 
which  characterize  chemical  affinity,  such  as  heat 
and  incandescence,  resulting  in  the  formation  of  sub- 
stances having  a  definite  composition,  distinct  crys- 
talline form,  and  properties  differing  from  those  of 
their  constituents. 

When  a  piece  of  clean  sodium  is  rubbed  in  a  mortar 
with  dry  mercury,  the  former  dissolves,  and  a  pecu- 
liar seething  sound,  resembling  that  caused  by  the 
immersion  of  a  hot  body  in  water,  is  produced,  due 
to  the  evolution  of  heat  which  accompanies  the  com- 
bination, the  mercury  rising  rapidly  in  temperature 
as  the  pieces  of  sodium  are  added.     As  the  mercury 

34 


ALLOYS.  35 

cools,  the  resulting  alloy,  which  is  brilliantly  white, 
crystallizes  in  long,  needle-like  forms  from  the  middle 
of  the  liquid,  and  the  excess  of  mercury  may  be 
poured  off. 

Alloys  are  generally  harder  and  more  fusible  than 
the  metals  of  which  they  are  formed,  and  as  many 
metals  are  unfit  in  the  pure  state  for  use  in  the  me- 
chanic arts,  owing  to  extreme  softness  or  high  fusing- 
point,  these  properties  are  modified  to  suit  various 
requirements  by  the  admixture  of  other  metals. 
Thus,  as  a  base  for  an  artificial  denture,  pure  gold 
would  be  too  soft  to  withstand,  without  bending,  the 
force  to  which  the  fixture  would  be  exposed  during 
mastication,  but  by  the  addition  of  sufficient  copper 
and  silver  to  reduce  the  gold  to  .750  (18  carats)  the 
necessary  rigidity  may  be  obtained  without  materially 
affecting  the  other  properties.  Again,  it  is  often  de- 
sirable to  unite  several  pieces  of  the  same  metal  or 
of  different  metals.  This  is  accomplished  by  means 
of  a  class  of  alloys  called  solders,  generally  formed  ot 
the  metal  upon  which  they  are  to  be  employed  with 
the  addition  of  some  other  metal  which  will  consider- 
ably lower  the  fusing- point  without  affecting  the  color, 
as  it  is  desirable  that  the  place  of  union  should  not 
be  noticeable.  For  example,  a  solder  suitable  for  use 
in  prosthetic  dentistry  should  fuse  at  a  much  lower 
temperature  than  the  plate  upon  which  it  is  to  be  used. 
Its  color  should  be  as  nearly  as  possible  the  same, 
and  what  is  even  more  important,  it  should  withstand 
the  action  of  the  fluids  of  the  mouth  nearly  as  well. 
These  properties  may  be  obtained  by  the  addition  of 
small  quantities  of  silver,  copper,  or  brass. 


36  DENTAL     METALLURGY. 

The  value  of  many  of  the  metals  for  industrial 
uses  is  very  greatly  enhanced  by  alloying.  Thus, 
copper,  which  is  unfit  for  casting  and  too  tough  for 
turning,  may,  by  the  addition  of  zinc,  be  rendered 
not  only  harder  and  more  elastic,  but  the  fusing-point 
of  the  resulting  compound  will  be  so  much  lower  than 
that  of  the  copper  alone  as  to  render  the  casting  of 
it  a  matter  of  no  great  difficulty,  while  at  the  same 
time  it  will  be  found  susceptible  of  being  turned  in 
the  lathe  with  facility. 

The  tendency  on  the  part  of  metals  to  unite  in 
definite  proportions  may  be  studied  in  connection 
with  platinum,  iridium,  gold,  rhodium,  ruthenium, 
and  silver,  when  fused  with  tin.  If  the  latter  metal 
is  in  excess,  after  cooling  a  metallic  ingot  is  obtained 
resembling  closely  the  tin  ;  but  by  the  action  of  strong 
hydrochloric  acid  upon  this  the  excess  of  tin  may 
be  dissolved,  leaving  crystals  of  a  definite  alloy  of 
the  tin  and  the  noble  metal,  which  cannot  be  further 
dissolved  by  the  same  acid,  but  are  soluble  in  nitro- 
hydrochloric  acid,  even  when  the  precious  metal  con- 
tained, whether  rhodium,  ruthenium,  or  iridium,  is  in 
the  free  state  absolutely  insoluble  by  that  agent. 

It  must  not,  however,  be  assumed  that  all  the  alloys 
employed  in  the  industrial  arts  are  the  result  of  defi- 
nite combination  dissolved  in  an  excess  of  one  of 
the  metals.  Many  combinations  are  capable  of  co- 
existing in  the  same  alloy.  This  may  be  demonstrated 
in  an  alloy  of  tin,  lead,  and  bismuth,  which  melts 
below  the  boiling-point  of  water.  Heated  to  250  C. , 
and  then  permitted  to  cool,  it  will  be  observed,  by  the 
assistance  of  the  thermometer,  that  the  fall  of  temper- 
ature   is    twice    distinctly   arrested.     The    cause    of 


ALLOYS.  37 

this  phenomenon  has  been  assumed  to  be  the  produc- 
tion in  the  compound  of  a  less  fusible  alloy,  which 
in  solidifying  evolves  heat,  and  thus  for  a  time  retards 
the  gradual  cooling  of  the  mass.  It  may,  therefore, 
be  assumed  that  true  chemical  combinations  may 
occur  between  two  metals,  notwithstanding  the  fact 
that  such  union  may  be  masked  by  excess  of  one  of 
the  constituents. 

Matthiesen,  in  an  elaborate  paper  on  the  subject, 
states  that ' '  an  alloy  may  be,  first,  a  solidified  solution 
of  one  metal  in  another  ;  second,  a  chemical  combina- 
tion ;  third,  a  mechanical  mixture  ;  or  fourth,  a  solid- 
ified solution  or  mechanical  mixture  of  two  or  all  of 
the  above."  In  simple  mechanical  mixture  of  two 
metals  there  is  often  a  tendency  to  separate.  This  is 
noticeable  in  some  alloys  of  silver  and  copper  by  an 
absence  of  perfect  homogeneity  in  the  ingot.  Again, 
some  of  the  metals  form  mixtures  so  decidedly 
mechanical  that  on  being  allowed  to  stand  after  fusing 
they  will  separate,  the  one  possessing  the  highest 
specific  gravity  settling  to  the  bottom.  This  may  be 
observed  when  lead  and  zinc  are  mixed.  Matthiesen, 
however,  found  that  the  lead  retains  1.6  percent,  of  the 
zinc,  while  the  zinc  retains  1.2  per  cent,  of  the  lead.* 

Density. — Theoretically,  it  might  be  supposed  that 
the  density  of  an  alloy  would  be  the  mean  of  its  con- 
stituents. Such,  however,  is  not  always  the  case,  as 
the  resulting  number  is  sometimes  equal  to,  or  greater 
or  less  than,  the  theoretical  mean.  The  density  of 
alloys  of  gold  and  silver  is  less  than  the  mean  of  the 
components,  in  consequence  of  expansion  ;  while  brass 
and  alloys  of  lead  and  antimony  vary  in  the  opposite 

*  Makins's  Metallurgy,  p.  62. 


38 


DENTAL    METALLURGY. 


direction,  through  a  condensation  of  their  constituents. 
But  in  the  formation  of  some  alloys  there  is  no  altera- 
tion of  volume,  and  the  density  of  such  will  corre- 
spond to  that  obtained  by  calculation  as  the  mean  of 
their  constituents. 

The  following  table,*  by  Th6nard,  gives  examples 
of  variations  of  density  in  alloys  :f 


Alloys  possessing  a  Greater  Specific 
Gravity  than  the  Mean  of  their 
Components. 

Gold  and  Zinc. 
"       "    Tin. 
"       "     Bismuth. 
"       "     Antimony. 
"       "    Cobalt. 
Silver  and  Zinc. 
"       "     Lead. 
"       "    Tin. 

"     Bismuth. 
"       "    Antimony. 
Copper  and  Zinc. 
"     Tin. 
"    Palladium. 
"    Bismuth. 
"  "    Antimony. 

Lead  and  Bismuth. 

"       "    Antimony. 
Platinum  and  Molybdenum. 
Palladium  and  Bismuth. 

Color  is  always  modified  by  alloying.  It  is  gener- 
ally such  as  might  be  expected  to  result  from  the 
mixture  of  the  metals  entering  into  the  formation  of 

♦Phillips   states  that  it  is    doubtful  whether    some    of  the    mixtures 
included  in  this  table  should  be  regarded  as  alloys. 
t  Phillips's  Metallurgy. 


Alloys  having  a  Specific  .'Gravity 
Inferior  to  the  Mean  of  their 
Components. 

Gold  and  Silver. 

"       "     Iron. 

"       "     Lead. 

"       "     Copper. 

"       "     Iridium. 

"      "     Nickel. 
Silver  and  Copper. 
Copper  and  Lead. 
Iron  and  Bismuth. 
"   Antimony. 

"       "   Lead. 
Tin  and   Lead. 

"      "    Palladium. 

"  "  Antimony. 
Nickel  and  Arsenic. 
Zinc  and  Antimony. 


ALLOYS.  39 

the  alloy.     There  are  a  few  instances,  however,  where 
it  is  different.     Thus,  three  parts  of  silver  to  seven  of 
gold  yields  a  green  alloy,  and  nickel,  added  to  brass, 
produces  an  alloy  of  silvery  whiteness. 

Malleability,  Ductility,  and  Tenacity. — These  prop- 
erties are  generally  much  changed  in  metals  by  alloy- 
ing ;  malleability  and  ductility  being  diminished,  and 
in  some  cases  entirely  destroyed,  even  in  the  combi- 
nation of  two  very  ductile  metals,  as  is  the  case  with 
gold  containing  a  small  quantity  of  lead,  ductility 
being  completely  lost.  Again,  gold  and  platinum, 
two  exceedingly  ductile  metals,  are  rendered  much 
harder  and  somewhat  elastic  by  admixture. 

The  union  of  a  brittle  and  a  ductile  metal  yields  a 
brittle  alloy.  According  to  Mr.  Makins,  antimony,  a 
metal  so  brittle  that  it  may  be  broken  up  in  a  mortar, 
when  added  to  gold  to  the  extent  of  j-^-q  part,  will 
make  the  gold  quite  unworkable. 

Tenacity  is  generally  increased  by  alloying.  The 
following  results  were  obtained  by  Matthiesen,  by 
employing  wires  of  the  same  gauge  and  noting  the 
weights  which  caused  their  rupture  before  and  after 
alloying  : 

LBS.  LBS. 

Copper,  unalloyed,  251030 ;  alloyed  with  12  perct.  Tin,  80  to  90 


Tin, 

<  < 

under  7  ; 

«< 

"          "         Copper,     7 

Lead, 

<  < 

"      7; 

<  < 

"Tin     ...     .        7 

Gold, 

<< 

20  to  25  ; 

i  < 

"    Copper   ...      70 

Silver, 

<  < 

45  to  50  ; 

<( 

"     Platinum     .  75  to  80 

Platinum, 

<  < 

45  to  50 ; 

Iron, 

1 1 

80  to  90  ; 

Steel  (I 

ron  alloyed  with 

Carbon)   ....     above  200 

Generally  speaking,   the  hardness  of  metals  is  in- 
creased  by    alloying   them.     A    familiar   instance   is 


40  DENTAL    METALLURGY. 

standard  gold  or  silver.  Neither  of  these  when  un- 
alloyed is  sufficiently  hard  to  resist  attrition  to  the 
degree  required  for  currency  ;  but  the  addition  of 
one-tenth  of  its  weight  of  copper  to  either  metal  in- 
creases its  hardness  to  the  required  point.  Ninety- 
four  parts  of  copper  with  six  parts  of  tin  form  an 
alloy  so  brittle  that  it  may  be  broken  with  a  hammer. 

Fusibility. — The  fusing-point  of  an  alloy  is  always 
lower  than  that  of  the  least  fusible  metal  entering  into 
the  composition  of  the  alloy.  Thus,  an  alloy  com- 
posed of  five  parts  of  bismuth,  three  of  lead,  and 
two  of  tin,  melts  at  91  °  C,  less  than  the  boiling-point 
of  water,  while  tin  alone  fuses  at  227. 8°  C. ,  and  lead 
at  3250  C,  and  the  addition  of  a  small  amount  of  cad- 
mium to  the  above  alloy  will  further  reduce  the  fusing- 
point  to  1400  F.,  or  6o°  C.  Lead  combined  with  a 
small  portion  of  silver  is  more  fusible  than  the  former 
in  a  state  of  purity,  and  an  alloy  may  be  formed  of  po- 
tassium and  sodium,  which  remains  fluid  at  ordinary 
temperatures  of  the  air. 

This  phenomenon  has  been  explained  by  Matthie- 
sen.  He  says  that  ' '  matter  in  the  solid  state  exhibits 
excess  of  attraction  over  repulsion,  while  in  the  liquid 
state  these  forces  are  balanced,  and  in  the  gaseous 
state  repulsion  predominates  over  attraction,  and 
similar  particles  of  matter  attract  each  other  more 
powerfully  than  dissimilar  particles  do.  The  attrac- 
tion subsisting  between  the  particles  of  a  mixture  will 
be  sooner  overcome  by  repulsion  than  will  the  attrac- 
tion in  the  case  of  a  homogeneous  body  ;  hence  mix- 
tures should  fuse  more  readily  than  their  constit- 
uents."* 

*Makins's  Metallurgy,  p.  65. 


ALLOYS. 


41 


Composition  of  Alloys. — A  statement  of  the  average 
proportions  in  which  metals  enter  into  the  best-known 
alloys,  the  composition  of  which  is  generally  very 
variable,  is  given  in  the  following  table  : 


Coinage  of  Gold  .  . 
Gold  Jewelry  and  Plate 
Silver  Coinage  .  .  . 
Silver  Vessels  .  .  . 
Silver  Jewelry  .  .  . 
Aluminum-Bronze 
Specula  of  Telescopes 
Pinchbeck  .... 
Brass     .... 


German  Silver 


Type-metal 


Bronze  Coins  and  Medals 

Bronze  Cannon  .     .     .    . 

Bronze  Bells 

Bronze  Cymbals      .     .     . 


(Gold  , 
<•  Copper 
J  Gold  , 
*•  Copper 
f  Silver  . 
<•  Copper 
/  Silver  . 
I  Copper 
f  Silver  . 
<■  Copper 
f  Copper 
<-  Aluminum 
f  Copper 
I  Tin 
f  Copper 
I  Zinc 
f  Copper 
I  Zinc 

{Copper 
Zinc 
Nickel 
f  Lead    . 
I  Antimony 

i  Copper 
Tin  . 
Zinc  . 
I  Copper 
I  Tin  . 
j  Copper 
I  Tin  . 
f  Copper 
I  Tin      . 


.  .  90 
.  .  10 
75  to  92 
25  to  8 
90 
10 
95 
5 
80 
20 
90  to  95 
10  to  5 
.     .     67 

•  •  33 
.  .  90 
.  .  10 
67  to  72 
28  to  33 
50 
25 
25 
80 
20 
94  to  96 
4  to  6 
1  to  5 
90 
10 
78 
22 
80 
20 


42  DENTAL     METALLURGY. 


English  Metal      . 

Pewter  .... 
Liquid  Measures. 
Plumbers'  Solder 


Tin 87 

Antimony    ...  8 

Bismuth  ....  1 

Copper    ....  4 

j  Tin 92 

I  Lead 8 

rTin 82 

iLead 18 

(Tin      .....  67 

I  Lead 33 


Alloys  used  as  plates  for  artificial  dentures  and 
those  constituting  solders  will  be  described  with  the 
metals  forming  their  bases. 

Decomposition. — When  the  alloy  contains  a  volatile 
metal,  like  zinc  or  mercury,  heat  decomposes  it,  but 
the  temperature  required  to  expel  the  last  trace  of 
the  volatile  metal  must  be  considerably  higher  than 
that  metal's  normal  temperature  of  ebullition.  If  the 
alloy  is  composed  of  a  noble  metal  and  zinc,  lead,  or 
tin,  and  it  is  desired  to  free  it  from  the  impurity,  this 
may  be  accomplished  by  exposure  to  a  high  tempera- 
ture, and  the  addition,  while  the  metal  is  fluid,  of  some 
substance  rich  in  oxygen,  such  as  potassium  nitrate. 
By  this  means  the  base  metal  is  converted  into  an 
oxid,  and  is  then  dissolved  and  held  in  solution  by  the 
borax,  which  should  be  used  as  a  flux  in  the  crucible. 
Metals  in  combination  with  mercury  may  be  separated 
by  the  application  of  heat,  the  mercury  volatilizing  at 
6oo°  F.  In  the  case  of  particles  of  amalgam,  how- 
ever, the  temperature  to  which  the  pieces  are  exposed 
should  be  at  least  a  bright-red  heat. 

Influence  of  Constituent  Metals. — Mercury,  bismuth, 
tin,  and  cadmium  give  fusibility  to  alloys  into  which 
they  enter ;  tin  also  gives  hardness  and  tenacity  ;  lead 


ALLOYS.  43 

and  iron  give  hardness  ;  arsenic  and  antimony  render 
alloys  brittle. 

It  has  been  observed  that  phosphorus  and  arsenic, 
when  added  to  alloys  of  copper  and  tin,  have  the  power 
of  deoxidizing  or  eliminating  metallic  oxids,  which 
are  invariably  present.  The  well-known  phosphor- 
bronze  owes  its  closeness  of  grain  and  superior  tenacity 
to  the  addition  of  phosphorus,  and  it  is  claimed  that 
' '  when  arsenic  or  arsenical  compounds  are  made  to 
unite,  under  suitable  conditions,  with  alloys  of  copper 
and  tin,  known  as  bronze  or  gun-metal,  it  imparts  to 
them  several  remarkable  and,  for  many  purposes  in 
the  arts,  desirable  properties, — among  others  and  prin- 
cipal of  which  are  homogeneity,  hardness,  elasticity, 
greatly  increased  tensile  strength  and  toughness,  and 
a  peculiar  smoothness,  rendering  it  a  valuable  anti- 
friction metal  for  journal-bearings,"  etc. 

The  arsenical  compounds  of  alloys  of  copper  and 
tin  are  also  more  fluid  when  molten  than  are  other 
known  alloys  of  copper  and  tin, — a  property  which 
renders  them  capable  of  filling  out  sharply  and  with- 
out flaws  the  most  intricate  molds. 

Liquation. — The  constituents  of  an  alloy  heated 
gradually  to  near  its  point  of  fusion  frequently  unite 
to  form  new  compounds,  and  if  the  fluid  portion  is 
poured  off,  there  remains  a  solid  alloy  less  fusible  than 
the  original.  Copper  is  separated  from  silver  by  this 
process.  In  bars  of  silver  alloyed  with  copper,  a 
curious  tendency  on  the  part  of  the  latter  to  separate 
and  aggregate  at  the  edges,  as  the  fused  mass  assumes 
the  solid  form,  has  been  observed.  Mr.  Makins  states 
that,  as  a  result  of  careful  examination  of  Mexican 
dollars   and    crown    pieces,    he   found   the    variation 


44  DENTAL   METALLURGY. 

between  the  center  and  edges  to  range  in  the  former 
from  one  to  six,  and  in  the  latter  from  one  to  four 
milligrammes.  He  gives  as  the  average  of  a  number 
of  experiments  on  twenty-four  crown  pieces  a  mean 
variation  of  two  milligrammes,  and  as  the  quality  in 
which  the  greatest  tendency  to  separate  is  shown  that 
of  900  parts  of  silver  to  100  of  copper.* 

Temper. — Modified  conditions  of  hardness  and  elas- 
ticity of  a  metal,  it  has  been  shown,  may  be  obtained 
by  admixture  with  other  metals  and  by  sudden  varia- 
tions of  temperature,  as  in  the  case  of  the  alloy  of  94 
parts  of  copper  and  6  of  tin,  which  forms  a  bronze  so 
brittle  that  it  may,  when  heated  and  slowly  cooled,  be 
pulverized  with  a  hammer  ;  but  if,  on  the  contrary, 
it  is  cooled  rapidly,  by  immersion  in  cold  water,  it 
becomes  malleable.  The  treatment  of  iron  mixed  with 
carbon  (steel)  is  just  the  opposite,  the  greatest  degree 
of  hardness  being  attained  by  suddenly  cooling  the 
heated  mass. 

Preparation. — When  the  alloy  is  to  be  formed  of  a 
noble  and  one  or  more  of  the  base  metals,  the  former 
should  be  thoroughly  fused  first  ;  the  latter  is  then 
added,  and  the  whole  covered  with  charcoal,  to  prevent 
oxidation,  and  then  thoroughly  mixed  by  stirring  or 
agitating. 

When  it  is  designed  to  lower  the  fusing-point  of 
gold  or  silver  for  use  as  solders,  by  the  addition  of 
brass,  etc.,  the  precious  metal  should  first  be  thor- 
oughly fused  with  a  sufficient  quantity  of  borax ; 
the  brass,  in  the  convenient  form  of  wire,  should  then 
be  quickly  thrust  into  the  melted  gold  or  silver.     It 

will  almost  instantly  mix  with  the  melted  mass,  and 

— — 

*  Makins's  Metallurgy,  pp.  379,  380. 


ALLOYS.  45 

the  borax,  if  in  sufficient  quantity,  will  cover  the 
liquid  alloy,  and  thus  protect  from  oxidation  by  con- 
tact with  the  atmosphere. 

The  action  of  acids  upon  alloys  is  generally  more 
energetic  than  upon  a  simple  metal,  but  it  varies 
according  to  the  relative  amounts  of  their  constitu- 
ents. Silver  alloyed  with  a  large  proportion  of  gold  is 
protected  from  the  action  of  nitric  acid.  Sometimes, 
however,  the  reverse  of  this  is  seen,  and  metals 
which  are  totally  insoluble  in  certain  menstrua  are 
made  to  dissolve  in  them  by  the  addition  of  a  metal 
on  which  they  have  the  power  of  acting.  Thus,  plat- 
inum, although  of  itself  insoluble  in  nitric  acid,  may 
be  dissolved  by  it  when  sufficiently  alloyed  with  silver. 

Alloys  consisting  of  two  metals,  one  readily  oxi- 
dizable,  the  other  possessing  less  affinity  for  oxygen, 
may  be  readily  decomposed  by  the  combined  action 
of  heat  and  air.  In  this  case  the  former  metal  will  be 
rapidly  converted  into  an  oxid,  excepting  perhaps 
the  last  portions,  which  may  in  some  degree  be  pro- 
tected from  further  action  by  the  oxid  already  formed. 
This  increased  affinity  for  oxygen  exhibited  by  alloys 
is  probably  an  electrical  phenomenon,  the  studv  of 
which  belongs  rather  to  the  science  of  chemistry  than 
to  metallurgy.  For  further  light  on  this  subject  the 
student  may  refer  to  Fownes's,  Bloxam's,  or  other 
standard  works  on  chemistry. 


CHAPTER   V. 

AMALGAMS. 

AMALGAM — the  name  given  to  an  alloy  of  mer- 
cury   with    one    or    more    other    metals.     The 
amalgams  are  a  very  numerous    class  of  com- 
pounds, and  many  of  them  are  used  largely  in  the  arts. 

Some  amalgamations  are  formed  merely  by  contact 
of  the  metals,  and  are  accompanied  by  evolution  of 
heat ;  others  are  obtained  by  the  action  of  mercury 
on  a  salt  of  the  metal,  or  the  action  of  the  metal  on  a 
salt  of  mercury,  thus  developing  in  some  cases  a  weak 
electric  current. 

The  constituents  of  amalgam  compounds  are  not 
generally  held  together  by  strong  affinities,  hence 
many  of  them  may  be  decomposed  by  pressure,  and  all 
by  high  temperatures.  Tin  amalgam  is  used  for  '  'sil- 
vering' '  mirrors  ;  gold  and  silver  amalgams  in  gilding 
and  silvering  ;  while  amalgams  containing  gold,  silver, 
tin,  platinum,  and,  in  some  cases,  cadmium,  zinc, 
copper,  and  other  of  the  base  metals,  compounded 
according  to  many  different  formulae,  have  been  used 
very  extensively  in  dentistry.  An  amalgam  of  zinc 
and  tin  is  employed  for  the  rubbers  of  electrical 
machines. 

An  alloy  for  dental  amalgams  should  possess  the 
qualities   of   strength    and    sharpness   of    edge,    and 
46 


AMALGAMS.  47 

freedom  from  admixture  with  any  metal  favorable  to 
the  formation  of  soluble  salts  of  an  injurious  character 
in  the  mouth.  It  should  be  capable  of  maintaining  its 
color,  although  in  an  alloy  composed  of  several  dif- 
ferent metals  absolute  freedom  from  discoloration, 
under  the  conditions  to  which  an  amalgam  filling  is 
exposed,  cannot  readily  be  obtained.  It  should  also 
be  capable  of  retaining  its  shape,  as  the  tendency  on 
the  part  of  many  amalgams  to  assume  a  globular 
form  after  their  introduction,  thus  leaving  the  edges 
of  the  cavity  unprotected,  is  probably  a  frequent 
cause  of  failure  in  this  class  of  fillings. 

Undue  expansion,  although  not  so  likely  to  occur 
as  some  other  changes,  would  be  equally  a  source  of 
failure.  According  to  Mr.  Fletcher,  amalgams  of 
silver  and  mercury  expand,  sometimes  sufficiently  to 
split  a  tooth  ;  and  Mr.  Kirby  states  that  "  amalgams 
of  pure  silver,  either  the  precipitate  or  filings,  expand 
greatly."  With  a  suitable  instrument  for  measure- 
ment, he  was  able  to  determine  the  change  in  bulk 
of  such  an  amalgam,  in  which  he  found  the  extent  of 
expansion  to  reach  one-fortieth  of  its  diameter. 

Many  old  amalgam  fillings  have  the  appearance  of 
projecting  from  the  edges  of  the  cavity  as  though 
there  existed  some  force  behind  or  beneath  sufficient 
to  push  them  out.  There  seems  to  be  some  diversity 
of  opinion  respecting  the  cause  of  this,  and  while  by 
some  it  is  attributed  to  expansion,  others  believe  it 
to  be  due  to  contraction.  Favoring  the  latter  theory, 
Mr.  Fletcher  states  that  he  has  found  it  to  occur  only 
with  those  amalgams  which  are  known  to  shrink, 
and  he  suggests  that  the  plug  may  be  raised  or  forced 
out  by  the  "decomposition  of  tooth-substance  and  the 


48  DENTAL     METALLURGY. 

formation  of  gas  under  the  loosened  plug,  the  driving 
down  and  accumulation  of  food  underneath,  or  some 
similar  cause. "  It  is  evident  that  an  amalgam  liable 
to  contract  or  expand  to  a  marked  extent  is  not  to 
be  relied  upon  as  a  filling- material. 

Discoloration  of  dental  amalgams  depends  largely 
upon  the  formation  of  sulfids.  The  fluids  of  the 
mouth,  in  every  case  where  the  most  scrupulous 
cleanliness  is  not  observed,  may  be  said  to  contain 
sulfur  in  combination  with  hydrogen,  as  dihydric 
sulnd  (H2S),  resulting  from  decomposition  of  par- 
ticles of  food  having  a  lodgment  between  or  adhering 
to  the  teeth.  The  affinity  of  sulfur  for  both  silver 
and  mercury  is  so  active  that  we  may  reasonably 
assume  that  not  only  the  discoloration  of  amalgam 
fillings,  but  in  many  cases  their  failure  to  prevent  a 
recurrence  of  decay,  is  due  to  the  action  of  that 
element  upon  the  alloy.  Nor  is  it  safe,  in  compound- 
ing alloys  for  dental  amalgams,  to  depend  upon  the 
protecting  influence  of  metals  which  do  not  possess 
the  same  affinities,  such  as  gold  and  platinum  ;  for 
while  these  metals  individually  may  remain  wholly 
unaffected  by  contact  with  sulfur,  it  does  not  neces- 
sarily follow  that  their  presence  in  an  alloy  will 
secure  the  same  immunity  to  such  metals  as  silver 
and  mercury. 

There  are  doubtless  other  causes  for  the  discolora- 
tion of  amalgams,  some  of  them  purely  adventitious, 
depending  upon  the  administration  of  certain  reme- 
dies in  diseased,  abnormal  conditions  of  the  fluids  of 
the  mouth,  or  the  presence  of  vegetable  acids  in 
articles  of  food,  such  as  fruit. 

An    amalgam    filling    may    retain    its    integrity    of 


AMALGAMS.  49 

surface,  while,  at  the  same  time,  the  darkening  of 
the  tooth-substance  unmistakably  indicates  chemical 
action  at  its  peripheral  portion,  doubtless  due  to  im- 
perfect adaptation,  favoring  the  ingress  of  the  eroding 
agent.  It  appears  that  almost  any  amalgam  filling 
may  be  kept  bright  by  friction,  whether  of  the  brush 
or  from  mastication,  and  it  seems  equally  certain  that 
all  such  fillings  will  blacken  if  the  position  which 
they  occupy  protects  them  entirely  from  friction. 
Again,  an  amalgam  filling  may  retain  its  original 
color  and  brightness  of  surface,  and  yet  not  protect 
the  tooth  ;  and,  conversely,  a  filling  of  this  class  may 
exhibit  a  great  degree  of  surface-discoloration  and 
yet  fully  preserve  the  tooth  from  further  decay, 
peripheral  discoloration  being  much  the  worse  con- 
dition of  the  two. 

In  a  number  of  experiments  made  with  some  of 
the  well-known  amalgams,  such  as  Townsend's,  Ar- 
lington's, "Standard  Alloy,"  Lawrence's,  Walker's, 
etc.,  as  well  as  with  some  of  higher  grades,  it 
was  found  that  with  care  in  using  the  proper  quan- 
tity of  mercury,  and  in  packing  the  mass  into  clean 
glass  tubes,  subsequently  filled  with  colored  fluid  and 
closely  sealed,  there  was  after  several  weeks  not  the 
slightest  apparent  leakage  ;  and  yet,  when  the  same 
tubes  were  thrown  into  a  solution  of  sulfuretted 
hydrogen,  the  edges  were  attacked,  and  marked  dis- 
coloration occurred  at  the  periphery  of  the  fillings, 
while  the  surface  directly  exposed  to  the  action  of 
the  sulfur,  and  not  in  contact  with  the  glass,  was 
but  slightly  clouded.  The  edges  had  the  appearance 
of  having  been  eroded,  as  by  an  acid.  This  exper- 
iment is  not  merely  speculative,  as  it  is  simply  filling  a 


50  DENTAL   METALLURGY. 

cavity  with  amalgam  and  then  exposing  it  to  sulfur  in 
the  form  usually  found  in  the  mouth.  The  result  is 
precisely  similar  to  that  which  is  observed  in  the  great 
majority  of  amalgam  fillings  in  actual  service.  It 
would  seem,  however,  that  the  purely  theoretical  test 
of  covering  the  plug  with  colored  fluids,  such  as  indigo, 
blue  ink,  etc.,  known  as  the  "  color- test,"  is  not  to 
be  relied  upon  in  testing  peripheral  adaptation,  for  all 
the  plugs  used  in  these  experiments  had  apparently 
excluded  the  passage  of  a  solution  of  indigo  or  ink. 
Yet,  that  they  did  not  perfectly  seal  the  tubes, 
although  introduced  under  very  favorable  conditions, 
was  plainly  shown  by  the  result,  which  indicated  that 
solutions  of  one  or  more  of  the  constituents  of  the 
alloy  had  taken  place,  accompanied  doubtless  with 
the  formation  of  new  compounds,  consisting  of  sul- 
fids  of  silver  and  mercury,  and  in  some  instances, 
probably,  of  copper. 

Influence  of  Different  Metals  in  Dental  Alloys. — 
Tin  dissolves  very  easily  in  mercury,  but  the  alloy 
hardens  slowly  and  imperfectly.     Without  admixture 
with  other  metals  it  is  unfit  for  use  in  the  form  of 
an  amalgam  in  the  mouth.     It  is  also  well  known  to 
possess  a  tendency  to  draw  away  from  the  edges  of 
any  cavity  into  which  it  may  be  packed,  and  to  as- 
sume a  globular  form,   and  it   never  becomes   suffi- 
ciently hard  to  answer  the  requirements  of  a  filling- 
material.      Mixed   with   other  metals,  tin   serves   to 
facilitate  amalgamation  and  to  afford  different  degrees 
of  plasticity.     Between  mercury  and  silver  the  affin- 
ity is  but  slight.     By  the  addition  of  tin,   however, 
union  is  facilitated. 

Silver  apparently  unites  very  readily  with  mercury. 


AMALGAMS.  5 1 

Yet  it  will  be  found,  upon  close  examination,  that 
complete  solution  of  pure  silver  filings  will  not  take 
place  until  after  long  contact,  unless  the  silver  is  in 
a  finely  divided  state  and  the  mercury  heated.  Under 
these  circumstances,  amalgamation  will  be  more 
readily  accomplished.  Amalgams  of  silver  and  mer- 
cury alone  are  said  to  expand. 

Silver  with  tin  added  forms  an  alloy  very  white  in 
appearance,  but  more  easily  oxidized  than  either  of 
the  constituents  ;  mixed  with  mercury,  an  exceed- 
ingly unctuous  and  plastic  amalgam  is  obtained,  but 
it  is  somewhat  slow  in  hardening.  There  is  much 
diversity  of  opinion  in  regard  to  the  contraction  and 
expansion  of  this  alloy.  Dr.  Hitchcock  and  Mr. 
Tomes  both  claim  contraction*  for  it,  while  Mr.  Kirby 
states  that  an  ' '  amalgam  of  an  alloy  of  3  parts  of 
silver  and  2  of  tin  contracts  slightly  at  first,  but 
finally  expands  about  ^ou."t  These  variable  results 
may  depend  upon  the  quantity  of  mercury  employed. 
The  author  is  satisfied  that  when  an  alloy  of  silver  and 
tin  is  used  no  excess  of  mercury  should  be  present,  and 
when  this  precaution  has  been  carefully  observed  he 
has  found  the  results  to  be  quite  as  good  as  those 
obtained  with  the  alloys  of  the  higher  grades.  Thus, 
500  milligrammes^  of  Arrington's  amalgam,  com- 
posed^ of  silver  40  per  cent.,  tin  60  per  cent.,  mixed 
with  160  milligrammes  of  mercury,  withstood  the  sul- 


*  Transactions  New  York  Odontological  Society,  1874. 

tlbid. 

X  The  author  has  used  the  metric  system  of  weights  in  recording  his  own 
experiments.  Where  data  have  been  obtained  from  others,  he  has  used  the 
figures  furnished  by  them,  and  in  quoting  other  authorities  he  has  not  felt 
at  liberty  to  change  the  text  in  any  way. 

g  Hitchcock,  Trans.  N.  Y.  Odont.  Soc,  1874. 


52  DENTAL     METALLURGY. 

furetted  hydrogen  test  quite  as  well  as  those  containing 
gold  and  platinum.  It  should  be  borne  in  mind  that 
alloys  composed  of  tin  and  silver  require  much  less 
mercury  to  render  them  plastic  than  those  contain- 
ing gold  and  platinum  in  addition. 

Probably  the  most  unfavorable  property  observed 
in  an  alloy  of  silver  and  tin  is  slowness  in  hardening, 
which  favors  the  ingress  of  fluids  by  capillary  force. 
The  direct  influence  which  silver  exerts  upon  an 
amalgam  of  tin  and  mercury  is  to  lessen  the  tendency 
to  assume  the  spheroidal  form,  and  to  facilitate  set- 
ting. In  this  respect  its  action  is  similar  to  that  of 
gold ;  but  while  the  latter  further  lessens  these 
injurious  tendencies,  and  confers  similar  properties, 
it  cannot  be  made  to  supersede  silver.  In  my  experi- 
ments with  these  curious  compounds  I  formed  an 
alloy  consisting  of 

Gold 500  milligrammes 

Platinum     ....         500  " 

Silver 2000 

Tin 2500 

An  amalgam  of  this  alloy  hardened  almost  instantly, 
so  that  a  filling  of  it  might  be  inserted  and  finished 
at  one  sitting.  For  the  sake  of  experiment,  another 
ingot  was  made,  from  which  the  silver  was  omitted. 
The  result  was  an  exceedingly  brittle  alloy,  which 
could  only  be  made  to  unite  with  mercury  by  heat- 
ing, and  even  then  with  difficulty,  and  it  did  not 
harden  sufficiently  to  be  of  any  use  as  a  filling- ma- 
terial. Thus  it  will  be  seen  that  silver  fills  an  impor- 
tant place  in  dental  alloys,  since  without  its  presence 
amalgamation  becomes  difficult,  and  rapidity  in  hard- 
ening is  not  secured. 


AMALGAMS.  53 

Gold  combines  with  mercury  at  all  temperatures, 
but  for  rapid  amalgamation  an  elevation  of  tempera- 
ture is  required,  and  the  process  is  accelerated  if  the 
gold  is  in  a  state  of  fine  division.  With  mercury  alone 
it  does  not  harden  well ;  added  to  tin,  it  to  a  certain 
extent  facilitates  setting,  but  does  not  harden  suffi- 
ciently for  use  in  the  mouth,  though  it  prevents  the 
tendency  to  draw  away  from  the  edges.  But  it  is  when 
added  to  an  alloy  of  tin  and  silver  that  the  greatest 
benefit  is  derived  from  its  presence.  I  found  that  an 
alloy  consisting  of 

Gold 500  milligrammes, 

Silver         ....       2000  " 

Tin 2500 

when  mixed  with  mercury  in  the  proportion  of  500 
milligrammes  of  the  alloy  to  250  milligrammes  of 
mercury,  retained  its  sharpness  of  edge,  hardened  well 
in  a  few  minutes,  and  apparently  filled  all  the  require- 
ments of  a  dental  amalgam.  The  author  is  aware  of 
the  statement  which  has  been  made  that  gold  added  to 
an  alloy  of  tin  and  silver  retards  hardening,  but  this  is 
doubtless  an  error,  and  the  presence  of  an  excess  of 
mercury  is  the  real  cause  of  the  tardiness  in  setting. 

Platinum,  in  the  usual  form  of  plate  or  wire,  does 
not  readily  unite  with  mercury.  A  very  smooth  and 
plastic  amalgam  may,  however,  be  formed  by  rubbing 
some  finely-divided  platinum,  such  as  is  obtained  by 
precipitation,  with  mercury  in  a  heated  mortar. 

An  amalgam  composed  of  platinum  and  mercury 
alone  does  not  harden  well.  The  properties  of  an 
alloy  of  tin  and  silver  are  also  greatly  impaired  by 
the  addition  of  platinum  in  any  considerable  quantity. 


54  DENTAL    METALLURGY. 

The  author  found  an  alloy  consisting  of  tin  2500 
milligrammes,  platinum  500  milligrammes,  to  be  ex- 
ceedingly brittle,  and  with  so  little  affinity  for  mercury 
that  amalgamation  was  only  accomplished  by  eleva- 
tion of  temperature  and  much  rubbing,  while  the 
property  of  setting  was  almost  entirely  lost. 

If  platinum  be  added  to  an  alloy  of  gold  and  tin, 
the  same  negative  results  are  observed.  When  com- 
bined with  tin,  silver,  and  gold,  however,  the  influence 
of  platinum  becomes  apparent.  With  the  proper 
proportion  of  mercury,  it  seems  to  confer  upon  such 
an  alloy  the  property  of  almost  instantly  setting,  as 
well  as  much  greater  hardness.  Thus,  it  will  be  seen 
that  the  qualities  claimed  for  platinum  per  se  belong  in 
reality  to  the  combination  of  tin,  silver,  gold,  and  plat- 
inum with  mercury,  since  if  either  one  of  the  others 
is  omitted  the  platinum  does  not  even  remain  passive, 
but  actually  by  its  presence  causes  marked  deteriora- 
tion of  the  qualities  essential  in  a  dental  amalgam. 

Alloys  containing  platinum  amalgamate  less  readily 
than  those  wherefrom  it  is  absent ;  yet  when  union  has 
once  begun  they  seem  to  require  a  larger  quantity  of 
mercury  to  render  them  plastic.  By  careful  exper- 
iment, the  author  found  that  with  an  alloy  of 

Platinum  ....  500  milligrammes, 

Gold  ....  500 

Silver  ....  2000 

Tin  ....  2500 

the  smallest  amount  of  mercury  which  could  be  em- 
ployed without  impairing  the  strength  and  general 
working  qualities  of  the  amalgam,  was  300  milli- 
grammes to  500  milligrammes  of  the  alloy  ;  while 
with  another  ingot  composed  of 


AMALGAMS.  55 

Gold 500  milligrammes, 

Silver         ....     2000 
Tin 2500 

160   milligrammes    of  mercury  to    500    of  the    alloy 
afforded  a  perfectly  good  result. 

The  proper  quantity  of  mercury  should  be  ascer- 
tained by  careful  experiment,  as  the  statements  of 
manufacturers  or  venders  are  not  always  reliable. 

In  order  to  ascertain  the  proportion  of  mercury  re- 
quired by  different  alloys,  a  small  quantity  of  the 
latter  should  be  taken,  say  one  gramme,  and,  after 
weighing,  the  mercury  may  be  carefully  added  and 
mixed  by  rubbing  until  the  mass  assumes  a  semi- 
coherent  state.  It  should  then  be  weighed  again,  to 
determine  accurately  how  much  mercury  is  present. 
It  may  then  be  introduced  into  a  glass  tube  and  con- 
densed by  means  of  instruments  slightly  warmed. 
Should  the  proportions  not  be  correct,  another  trial 
may  be  made,  and  the  quantity  of  mercury  increased 
or  diminished  as  indicated  by  the  results  of  the  first 
experiment. 

The  quantity  of  mercury  required  by  each  alloy  is 
probably  definite,  so  that  a  tolerably  accurate  approxi- 
mation of  the  composition  of  an  alloy  should  be  ascer- 
tained by  carefully  noting  the  required  proportions  of 
one  to  the  other. 

Different  methods  are  employed  for  the  attainment 
of  this  object.  Probably  the  most  common  is  to  mix 
the  alloy  with  a  large  excess  of  mercury,  and  then  to 
express  the  surplus  of  mercury  either  by  compression 
with  the  fingers  or  through  the  pores  of  a  piece  of 
chamois-leather.  The  first  involves  the  loss  of  some 
of  the  alloy,  which  is  carried  away  with  the  surplus 


56  DENTAL     METALLURGY. 

of  mercury,   and    neither  is  to    be  relied    upon  as  a 
means  of  excluding  an  excess  of  mercury. 

There  are  also  several  methods  employed  in  mixing 
amalgams.  Probably  the  most  common  one  consists 
in  simply  rubbing  the  alloy  and  the  mercury  together 
in  the  palm  of  the  hand.  This  is  certainly  the  most 
expeditious,  though  not  a  very  neat  way,  and  much 
has  been  said  about  manifestations  of  the  physiologi- 
cal effects  of  mercury  following  a  long  continuance  of 
the  practice.  The  author  has  made  considerable  effort 
to  ascertain  the  correctness  of  this  theory,  in  view  of 
the  fact  that  certain  phases  of  ill- health  have  been 
attributed  to  absorption  of  mercury  through  this 
method  of  mixing  amalgams,  but  he  has  been  unable 
to  trace  a  single  case  of  ptyalism  or  any  other  well- 
marked  sign  of  mercurial  poisoning  to  this  cause. 
It  would  be  well  to  remember,  however,  that  the 
active  properties  of  mercury  are  developed  by  a 
state  of  fine  division,  and  that  there  is  nothing  un- 
reasonable in  the  theory  that  mercury,  highly  com- 
minuted by  rubbing  in  the  palm  of  the  hand,  may 
find  its  way  into  the  system  and  produce  con- 
stitutional disturbance.  In  view  of  these  facts,  it 
would  be  a  proper  precaution  to  prevent  contact  of 
the  mercury  with  the  skin.  This  may  be  accom- 
plished by  covering  the  hand  with  a  piece  of  rubber- 
dam,  forming  a  sort  of  mitten,  leaving  the  fingers 
free  and  having  an  opening  for  the  thumb  to  pass 
through. 

Small  porcelain  and  glass  mortars  are  also  employed 
in  promoting  amalgamation,  but  they  do  not  effect 
the  desired  purpose  speedily,  in  consequence  of  the 
granules    of  the   alloy   becoming   burnished   by   the 


AMALGAMS.  57 

attrition    of  the   pestle.      Heating    the    mortar   will, 
however,  greatly  facilitate  union. 

Mr.  Fletcher  has  recently  called  attention  to  a 
simple  and  effective  method  of  mixing  amalgams. 
The  required  weight  of  filings  and  mercury  are  put 
into  a  long,  narrow  test-tube  or  bottle,  and  well 
shaken  for  a  few  seconds.  The  percussive  force 
brought  to  bear  upon  the  mass  promotes  prompt 
union. 

Some  difficulty  may  be  encountered  in  introducing 
amalgam  fillings  when  the  mercury  has  been  reduced 
to  the  minimum.  The  semi-coherent  mass,  almost  in 
the  form  of  a  powder,  is  not  easily  conveyed  to  the 
cavity,  especially  if  it  is  in  the  superior  arch.  Fletcher 
has  devised  a  sort  of  mold  by  which  amalgams  mixed 
so  dry  as  to  be  unmanageable  ordinarily  may  be 
rapidly  shaped  into  a  convenient,  workable  form. 
It  consists  of  a  cylinder,  into  which  the  semi- coherent 
mass  is  poured.  By  means  of  a  piston  the  powder  is 
compressed  into  disks  of  the  desired  thickness,  which 
may  be  introduced  into  the  cavity  and  solidly  com- 
pressed with  slightly  warmed  instruments.  It  is 
claimed  that  this  method  practically  does  away  with 
the  chief  objection  to  the  use  of  very  dry  amalgams. 
Should  the  amalgam  harden  and  become  unmanage- 
able before  the  completion  of  the  filling,  it  may  be 
rendered  plastic  and  cohesive  without  disturbing  the 
process  of  crystallization  or  the  property  of  setting, 
by  the  use  of  slightly  heated  instruments.  A  further 
addition  of  mercury  will  be  found  to  greatly  impair, 
if  not  destroy,  these  properties. 

Amalgams  may  be  regarded  as  chemical  compounds 
having    definite    proportions,    and    capable    of  being 

5 


58  DENTAL     METALLURGY. 

dissolved  in  an  excess  of  mercury,  in  which  condition, 
however,  they  will  no  longer  be  found  to  fulfill  the 
requirements  of  a  filling-material.  Hence,  the  opera- 
tor should  in  no  case  attempt  to  restore  the  plasticity 
of  an  amalgam  which  has  once  hardened,  by  a  further 
addition  of  mercury. 

Forming  Alloys  for  Amalgams. — The  putting  to- 
gether of  the  constituents  of  an  alloy  composed  of 
tin,  silver,  gold,  and  platinum,  is  a  matter  of  no  great 
difficulty,  as  it  does  not  require  an  extraordinary 
degree  of  heat,  and  it  may  be  easily  accomplished  in 
a  stove  or  furnace  such  as  is  usually  employed  for 
heating  or  cooking  purposes.  The  small  reverbera- 
tory  furnaces  devised  by  Mr.  Fletcher  will  be  found 
to  answer  the  purpose  admirably.  The  author  has 
used  one  successfully  in  a  large  number  of  melting 
operations.     Their  cost  is  only  about  fe.50  each. 

The  only  difficulty  likely  to  be  met  in  forming  an 
alloy  of  this  description  is  oxidation  of  the  tin,  and 
the  formation  of  certain  definite  compounds  having 
a  tendency  to  separate  from  the  mass,  thus  causing 
an  ingot  which  is  not  homogeneous.  Oxidation  of 
the  tin  may  take  place  at  the  instant  of  union  with 
the  platinum,  and  it  is  therefore  preferable  to  melt 
the  platinum  and  silver  together  first,  and  then  add 
the  tin  and  gold.  A  quantity  of  borax  should  be 
fused  in  the  crucible  before  the  metals  are  melted, 
the  objects  being  to  prevent  adhesion  of  the  alloy  to 
the  sides  of  the  crucible,  to  facilitate  pouring,  and 
to  dissolve  and  hold  in  solution  any  oxid  which  may 
be  present.  Lastly,  a  layer  of  broken  charcoal  should 
be  placed  over  the  mass  before  the  heating.  This 
will  perfectly  protect  it  from  oxidation. 


AMALGAMS.  59 

The  formation  of  definite  alloys,  it  has  been  shown, 
takes  place  with  the  gradual  cooling  of  the  mass  ; 
the  fusing-point  and  density  of  these  being  greater 
than  of  that  which  remains  fluid,  they  manifest  a  ten- 
dency to  settle  to  the  bottom  of  the  crucible  in  a  solid 
state,  and  in  some  cases  do  not  leave  the  crucible  in 
pouring.  Thus  the  ingot  may  not  possess  the  desired 
composition.  Again,  the  alloy  may  assume  a  semi- 
solid form,  floating  in  masses  in  the  more  fluid  portion, 
settling  at  the  sides  or  bottom  of  the  mold  at  the 
moment  of  pouring,  the  result  being  an  ingot  which 
is  not  uniform  in  composition,  in  consequence  of  which 
it  has  been  recommended  that  different  parts  of 
every  ingot  should  be  tested  ;  but  the  difficulty  may 
be  entirely  avoided  by  carrying  the  heat  to  the  point 
of  complete  fusion  and  pouring  while  still  very  hot, 
before  the  tendency  to  separate  is  developed. 

Oxidation  of  the  surface  from  contact  with  the  at- 
mosphere will  retard  amalgamation.  It  is  therefore 
better  not  to  reduce  the  entire  ingot  to  a  state  of  fine 
division.  It  will  be  found  to  unite  more  readily  with 
the  mercury,  if  freshly  filed  off  as  required  for  use. 
This  can  easily  be  effected  with  one  of  the  coarse  files 
sold  at  the  depots  as  vulcanite  files. 

Other  metals,  such  as  palladium,  copper,  cadmium, 
bismuth,  antimony,  and  zinc,  have  been  used  as  con- 
stituents of  amalgams. 

Copper  is  said  to  control  shrinkage,  while  it  in- 
creases the  tendency  to  discoloration.  It  is  also  be- 
lieved to  exert  a  preservative  influence  on  the  tooth- 
structure.  The  salts  of  copper  formed  around  amal- 
gam fillings  certainly  do  permeate  and  may  protect  the 
tooth-structure  from  further  decay,  but  too  much  re- 


60  DENTAL   METALLURGY. 

liance  should  not  be  placed  in  the  therapeutic  theory 
in  connection  with  this  class  of  amalgams.  It  is  ex- 
ceedingly difficult  to  determine  such  a  question. 
Amalgam  is  in  many  cases  placed  in  teeth  of  such 
good  quality  that  almost  any  filling-material  would 
answer  a  conservative  purpose,  and  the  examples 
which  are  often  presented  of  the  long  duration  of 
such  fillings  may  prove  nothing  beyond  the  density 
and  superior  quality  of  the  tooth-structure.  Careful 
and  intelligent  observation  and  experiment  alone  can 
satisfactorily  solve  a  question  of  this  kind. 

Brittleness  may  be  increased  or  diminished  accord- 
ing to  the  quantity  of  platinum  present,  and  the  re- 
sulting amalgam  will  exhibit  this  quality  equally  with 
the  alloy.  An  amalgam  filling  should  always  be 
strong  enough  to  retain  its  integrity  of  edge  under  the 
force  of  mastication.  A  formula  from  which  the 
author  has  obtained  very  good  results  is  as  follows  : 

Silver 40  grammes. 

Tin 60 

Gold 3 

Platinum 3 

Larger  percentages  of  gold  and  platinum  afford  no 
better  results,*  but,  on  the  contrary,  the  alloy  is  ren- 
dered more  brittle  thereby  ;  its  affinity  for  mercury  is 
lessened,  while  its  capacity  for  the  latter  is  increased, 
as  shown  under  the  head  of  "  Platinum." 

The  more  recent  experiments  with  amalgams  seem 

*  Under  the  assumption  that  amalgams  are  benefited  in  proportion  to 
the  amount  of  gold  contained,  Dr.  W.  G.  A.  Bonwill  recommended  that  from 
10  to  20  per  cent,  of  gold  be  dissolved  in  the  mercury  used  with  the 
alloy.  Dr.  Bonwill  has,  however,  ascertained  by  experiment  that  the  above 
percentage  was  excessive,  and  when  carried  beyond  a  certain  limit  gold 
ceases  to  be  useful  to  the  amalgam. 


AMALGAMS.  6 1 

to  favor  the  substitution  of  zinc  for  platinum.  The 
great  improvement  in  amalgam  filling- materials  which 
was  expected  from  the  use  of  platinum  has  not  been 
realized,  but  on  the  other  hand  alloys  containing  zinc 
instead  of  platinum  have  exhibited  better  qualities 
than  had  previously  been  attained.  Soon  after  the 
publication  of  the  first  edition  of  this  work,  the  author 
changed  the  above  formula,  using  zinc  instead  of 
platinum,  and  the  general  working  qualities  of  the 
amalgam  seemed  to  be  improved  by  the  alteration. 
A  number  of  fillings  made  with  it  at  the  time  have 
since  been  frequently  examined,  and  in  maintenance 
of  peripheral  integrity  and  color  have  been  in  every 
respect  satisfactory. 

Since  1878  Dr.  Louis  Jack  has  conducted  a  series 
of  careful  experiments  in  the  compounding  of  amal- 
gams, in  which  he  has  kept  accurate  notes  of  the  work- 
ing qualities  of  each  specimen  and  of  the  general  re- 
sults of  each  as  developed  by  time.  His  first  formula 
was  : 

No.  1. — Gold yz  oz. 

Silver 2       " 

Tin 3       " 

Platinum %    " 

This  was  gradually  modified  in  its  proportions,  until 
in  1885  the  formula  became  : 

No.  4. — Gold x/z  oz. 

Silver 1%    " 

Tin 3X    " 

Platinum -3      dwts. 

The  platinum  still  appeared  to  retard  amalgama- 
tion, and  cause  blackening  of  the  exposed  surface  of 
fillings.     A  new  formula  was   adopted,    from   which 


62  DENTAL   METALLURGY. 

platinum  was  excluded,  and  pure  zinc  used  in  its  place, 
as  follows  : 

No.  5.— Gold yz  oz. 

Silver i^f   " 

Tin 3X    " 

Zinc  ......  3      dwts. 

Dr.  Jack  states  that  fillings  of  formula  No.  5  have 
shown  great  durability,  clearly  maintained  edge 
strength,  and  sufficient  hardness  to  resist  attrition  on 
masticating  surfaces,  but  they  became  so  much  dis- 
colored by  the  action  of  hydrogen  sulfid  that  he 
gradually  increased  the  quantity  of  gold,  and  slightly 
changed  the  ratio  of  the  silver  and  tin,  until  formula 
No.  8  was  reached  : 

No.  8.— Gold  .        .        .        .        .        .14  dwts. 

Silver 2  oz. 

Tin 3   " 

Zinc 3  dwts. 

Fillings  in  the  mouth  composed  of  this  alloy  show 
but  slight  surface  discoloration,  and  come  very  near 
to  meeting  all  the  requirements  of  a  good  filling- 
material  if  proper  precautions  in  mixing  are  observed. 
To  amalgamate  readily,  the  alloy  should  be  freshly 
filed,  and  with  the  minimum  amount  of  mercury, 
which  may  be  readily  ascertained  by  experiment, 
union  of  the  two  may  be  obtained  by  violently  shaking 
them  in  a  test-tube,  until  the  mass  becomes  somewhat 
plastic.  It  is  then  kneaded  in  a  chamois-skin  or  nap- 
kin, this  being  done  to  prevent  the  oil  and  moisture 
of  the  hand  from  becoming  incorporated  with  the 
amalgam,  which  Dr.  Jack  considers  of  great  impor- 
tance. The  consistence  of  the  mass  should  be  such 
that  it  is  inclined  to  be  friable.     The  material  is  then 


AMALGAMS.  63 

cut  into  pieces  and  packed,  piece  by  piece,  either  with 
the  automatic  mallet  with  broad,  flat  points,  or  with 
heated  pluggers.  Should  free  mercury  appear  on  the 
surface,  this  should  be  scraped  away,  and  the  filling 
completed  with  dry  pellets. 

Dr.  Jack  states  that  it  is  doubtful  what  part  the  zinc 
plays  as  an  ingredient  of  amalgams,  but  he  is  now 
engaged  in  a  series  of  experiments  to  determine  its 
true  value  by  omitting  it  entirely  from  formula  No.  5 
and  carefully  noting  the  effect. 

It  is  of  importance  that  all  the  different  metals  taking 
part  in  the  formation  of  amalgam  alloys  should  be 
pure,  and  in  mixing  the  alloy  for  introduction  into  the 
cavity  the  utmost  dryness  and  cleanliness  be  main- 
tained ;  and  of  course  it  is  understood  that  no  test- 
filling  of  amalgam  can  be  fairly  made  unless  the 
thorough  preparation  of  the  cavity  and  all  other 
conditions  of  successful  filling  have  been  carefully  per- 
formed. 

Among  the  important  facts  elicited  by  Dr.  Jack's 
investigations  are  that  platinum  as  a  constituent  in 
amalgam  alloys  is  of  questionable  use,  and  that  a 
larger  proportion  of  gold  than  is  given  in  formula  No. 
8  is  impracticable,  since  an  increase  beyond  that 
amount  prevents  setting,  and  the  mass  remains  too 
soft  for  a  filling-material. 

The  following  are  the  different  formulas  employed 
by  Dr.  Jack  in  his  investigations  since  1878  : 


No.  1. 

Xo. 

3- 

Gold 

%   Q>7.. 

Gold 

•       lA  oz 

Silver 

.       2         " 

Silver 

. 

.     2      " 

Tin  . 

•     3      " 

Tin     . 

•     3      " 

Platinum 

. 

•       X   " 

Platinum 

•     3#    c 

dwts. 


64 


DENTAL  METALLURGY. 


No.  4. 


Gold  . 
Silver 
Tin     . 
Platinum 


Gold  . 
Silver 
Tin     . 
Pure  Zinc 


oz. 


i& 


No.  5. 


3/4 

3  dwts. 


Xz    oz. 
3    dwts. 


Has  always  shown  good 
results,  but  exhibits  surface 
discoloration. 


No.  6. 


Gold  . 
Silver 
Tin     . 
Pure  Zinc 


Gold. 
Silver 
Tin  . 
Pure  Zinc 


No.  7. 


v-i  oz. 

4  dwts. 


12    dwts. 
\}i  oz. 

3X   " 
8  dwts. 


No.  8. 


Gold 
Silver 
Tin     . 
Pure  Zinc 


14  dwts. 

2  oz. 

3  " 

3  dwts. 


Results  uniformly  good. 


No.  9. 


Gold  . 
Silver 
Tin  . 
Pure  Zinc 


Gold 
Silver 
Tin     . 
Pure  Zinc 


No.  10. 


14    dwts. 
1%  oz. 

*yA  « 

3  dwts. 


16  dwts. 

2  oz. 

3  " 

3  dwts. 


Too  soft,  and  suffered  some 
loss  by  surface  waste. 


No.  11. 


Gold  . 
Silver 
Tin     . 


y.  oz. 


From  actual  experience  with  Nos.  5  and  8,  the  author 
can  testify  to  the  excellence  of  their  general  working 
qualities  and  their  durability.  They  afford,  however, 
very  quick  amalgams,  and  should  not  be  mixed  with 
mercury  until  the  cavity  is  quite  ready  for  the  recep- 
tion of  the  filling  ;  or,  if  found  to  set  too  quickly,  that 
quality  may  be  modified  by  using  less  gold  in  the 
formula. 


AMALGAMS.  65 

Palladium  and  mercury  are  said  to  make  a  very 
good  filling.  According  to  Mr.  Thomas  Fletcher, 
however,  all  alloys  into  which  palladium  enters  as  a 
constituent  are  utterly  worthless  ;  combined  with  tin 
and  silver,  it  has  been  found  to  be  unsatisfactory.  Mr. 
Fletcher  gives  the  results  of  a  number  of  experiments 
with  tin,  silver,  and  palladium.*  "The  alloys  given 
below  were  made  from  chemically  pure  metals.  They 
were  melted  first  at  a  high  temperature,  under  a  layer 
of  charcoal,  in  a  clay  crucible,  with  constant  stirring. 
They  were  then  poured  quickly  into  a  thick  and  cold 
iron  ingot-mold  ;  broken  up  and  remelted  three  times, 
to  insure  uniformity  as  far  as  possible. 

Pd.  1,  Ag.  5.  Result  powdery  and  unmanage- 
able. 

Pd.  1,  Ag.  5,  Sn.  1.  Result  ditto.  Both  readily 
combined  with  Hg. 

Pd.  1,  Ag.  5,  Sn.  2.     Result  same  as  above. 

Pd.  1,  Ag.  5,  Sn.  3.  Result  very  dirty  to  mix  ; 
makes  a  leaky  plug. 

Pd.  1,  Ag.  5,  Sn.  6.     Result  similar  to  last. 

Pd.  1,  Ag.  3,  Sn.  5.  Result  very  dirty  ;  does  not 
combine  properly  with  mercury. 

Pd.   1,  Ag.  6,  Sn.  5,  Au.  1.    Result  similar  to  last. 

Pd.  1,  Sn.  4.  Result  very  dirty  ;  does  not  set  at 
all." 

In  consequence  of  the  uniformly  bad  results  of  these 
combinations,  Mr.  Fletcher  discontinued  further  ex- 
perimentation in  palladium  alloys  for  dental  amal- 
gams. 

The  claim,  however,  has  been  made  for  palladium,  f 

*  British  Journal  of  Dental  Science. 

tDr.    Bogue,    in    Proceedings   of   New    York    Odontological    Society, 
Dental  Cosmos,  1884,  p.  403. 


66  DENTAL   METALLURGY. 

that  when  used  as  a  single  metal  amalgamated  with 
mercury,  like  Sullivan's  amalgam  (copper  and  mer- 
cury, see  chapter  on  "  Copper"),  it  does  not  change 
in  form  or  bulk,  or  discolor  the  teeth,  and  though  it 
turns  quite  black,  it  is  a  thoroughly  reliable  filling  pro- 
vided the  cavity  be  properly  prepared.  It  is  said  that 
the  union  of  palladium  and  mercury  is  a  true  chemical 
one,  and  is  accompanied  with  the  phenomena  usual  in 
such  cases  of  heat  and  incandescence. 

Qualitative  and  Quantitative  Examinations  of 
Amalgam  Alloys. — It  is  important  for  the  dentist 
employing  filling- materials  of  this  class  to  be  well  in- 
formed as  to  their  composition,  and  it  is  also  often  desir- 
able to  be  able  to  determine  the  composition  of  old 
amalgam  fillings,  the  constituents  of  which,  besides  tin 
and  silver,  are  unknown. 

With  a  compound  belonging  to  the  latter  class 
the  first  step,  after  weighing,  would  be  to  free  it 
from  mercury  by  heating  to  redness,  the  loss  of 
weight  at  the  second  weighing  indicating  the  amount 
of  mercury  which  was  present.  After  this  it  is 
to  be  treated  as  an  alloy,  one  gramme  of  which 
may  be  placed  in  a  test-tube  or  other  suitable  glass 
vessel,  and  acted  upon  by  a  sufficient  quantity  of 
chemically  pure  nitric  acid.  The  silver  is  dissolved 
and  converted  into  argentic  nitrate.  The  tin  is 
oxidized  and  becomes  metastannic  acid  (5Sn02, 
ioH20).  The  latter,  in  the  form  of  a  white  powder, 
settles  to  the  bottom  of  the  vessel.  If  gold  forms 
part  of  the  alloy,  it  will  be  recognized,  even  in  the 
smallest  quantity,  by  the  very  decided  purple  color 
which  it  imparts  to  the  precipitate,  due  to  the  for- 
mation of    purple    of    Cassius.     Platinum    may    be 


AMALGAMS.  67 

readily  detected  by  the  presence  in  the  test-tube  of 
the  finely  divided  metal,  quite  black  in  color. 

At  this  point  in  the  experiment,  if  neither  gold  nor 
platinum  is  present,  the  quantitative  estimation  of 
the  alloy  is  not  difficult,  and  may  be  accomplished  as 
follows  :  The  solution  should  be  rendered  neutral  by 
evaporation,  and  diluted  with  a  large  quantity  of  dis- 
tilled water.  The  oxidized  tin  will  be  found  at  the 
bottom  of  the  vessel.  After  pouring  off  the  solution, 
this  should  be  washed,  dried,  and  rendered  anhydrous 
by  heating  to  redness,  when  it  is  ready  for  weighing, 
every  100  parts  indicating  78.66  of  tin.  Returning 
to  the  solution,  the  silver  is  next  precipitated  by 
sodium  chlorid  or  hydrochloric  acid,  and  may  be 
collected  by  filtering. 

If  cadmium  is  present  in  the  remaining  solution, 
sulfuretted  hydrogen  will  throw  it  down  in  the  form 
of  a  yellow  powder  (sulfid  of  cadmium),  the  color 
distinguishing  it  from  zinc  sulfid,  which  is  white. 
The  alkalies,  potassa,  soda,  and  ammonia  throw 
down  the  oxid  of  cadmium  as  a  white  hydrate. 
Amnionic  carbonate  also  produces  a  white  precipitate, 
which  is  insoluble  in  excess  of  the  precipitant.  The 
latter  quality  also  distinguishes  it  from  zinc,  which  is 
soluble  under  similar  conditions. 

Zinc  may  be  precipitated  from  a  solution  by  potas- 
sic  carbonate,  added  in  sufficient  quantity  to  decom- 
pose any  ammoniacal  salts,  if  present,  as  such  would 
prevent  the  precipitation  of  the  carbonate  of  zinc. 
The  whole  is  now  to  be  evaporated  to  dryness,  hot 
water  added,  the  zinc  salt  boiled,  collected  by  filter- 
ing, and  lastly,  after  washing,  ignited  to  drive  off  the 
carbonic  acid,  when  oxid  of  zinc  remains.     In  quan- 


68  DENTAL   METALLURGY. 

titative  estimation,  zinc  is  always  weighed  in  this  form, 
the  oxid  being  calculated  as  containing  80. 24  per  cent, 
of  the  metal. 

If  copper  be  present,  as  it  frequently  is  in  amalgam 
alloys,  it  will  be  instantly  detected  by  the  greenish 
hue  which  it  imparts  to  the  solution,  after  the  ' '  break- 
ing-up"  by  nitric  acid.  Indeed,  the  appearance  of 
the  contents  of  the  test-tube  will,  after  the  student  has 
acquired  some  experience,  serve  to  convey  a  pretty 
clear  idea  of  the  composition  of  the  alloy.  For  in- 
stance, take  one  gramme  of  Arrington's  alloy,  com- 
posed of  pure  tin  and  silver,  and  act  upon  it  with 
nitric  acid ;  the  contents  of  the  test-tube  will  be  a 
colorless  liquid  and  a  perfectly  white  powder.  In 
another  test-tube  dissolve  some  of  Lawrence's  alloy, 
and  the  nitrate,  instead  of  being  perfectly  colorless  as 
in  the  preceding  experiment,  will  show  a  decidedly 
green  tinge,  and  the  presence  of  copper  is  verified 
by  the  blue  color  developed  by  the  addition  of  am- 
monia. Gold  and  platinum  will  be  detected  in  alloys 
containing  these  metals  by  the  appearances  already 
described. 

The  presence  of  copper  having  been  verified,  that 
metal  may  be  precipitated  as  cupric  sulfid,  by  sul- 
furetted  hydrogen.  It  may  then  be  collected  by 
filtering,  and  after  washing  and  drying  may  be  oxi- 
dized by  nitric  acid,  and  again  be  precipitated  by 
potassa,  and  may  then  be  weighed  as  an  oxid. 
Should  copper  and  cadmium  both  be  present,  the 
precipitate  obtained  by  sulfuretted  hydrogen  will 
consist  of  sulfids  of  cadmium  and  copper.  Boiling 
in  dilute  sulfuric  acid,  however,  dissolves  the  cad- 
mium so  that  the  copper  may  be  collected  by  filtering, 


AMALGAMS.  69 

after  which  the  cadmium  may  be  again  thrown  down 
by  sulfuretted  hydrogen. 

The  merest  trace  of  copper  in  solution  may  also 
be  detected  by  placing  a  drop  of  the  latter  on  a  strip 
of  clean  platinum  foil  and  touching  it  with  a  point 
of  zinc.  A  spot  of  reduced  copper  will  instantly 
appear. 

For  the  quantitative  analysis  of  an  alloy  containing 
tin,  silver,  gold,  and  platinum,  the  first  step  should 
be  to  remove  the  tin  by  deflagration.  This  is  accom- 
plished by  placing  the  alloy,  after  weighing,  in  a  small 
crucible  with  some  borax,  and  heating  to  bright 
redness.  While  at  that  high  temperature  small  por- 
tions of  potassium  nitrate  are  added.  This  is  de- 
composed ;  its  oxygen  unites  with  the  tin,  converting 
it  into  stannic  oxid  (SnOa).  After  cooling,  the 
crucible  may  be  broken,  and  the  remaining  button, 
together  with  any  small  globules  which  may  adhere  to 
the  sides  of  the  crucible,  collected  and  weighed,  the 
loss  indicating  the  amount  of  tin  which  was  present. 
If  the  deflagration  has  been  thoroughly  performed, 
the  button  will  be  entirely  freed  of  tin,  and  it  then 
remains  to  separate  the  silver  from  the  gold  and 
platinum.  This  may  be  accomplished  either  by  nitric 
or  sulfuric  acid.  But,  as  the  former  will  dissolve  a 
considerable  portion  of  the  platinum  along  with  the 
silver,  sulfuric  acid,  which  does  not  thus  affect  the 
platinum,  affords  more  accurate  results,  and  is  the 
agent  usually  employed  in  parting  operations  where 
the  alloy  consists  largely  of  silver,  with  an  appreci- 
able percentage  of  platinum. 

The  button,    now  consisting   of  silver,   gold,   and 
platinum,   should  be  rolled  into   a  thin  ribbon,    and 


70  DENTAL     METALLURGY. 

then  placed  in  a  glass  or  platinum  vessel  with  at  least 
two  and  a  half  times  its  weight  of  concentrated  sul- 
furic acid.  This  is  boiled,  during  which  strong 
action  is  evinced  by  copious  disengagement  of  sul- 
furous  anhydrid,  and  the  silver  is  converted  into  a 
sulfate.  The  boiling  is  continued  until  all  the  silver  is 
dissolved,  when  the  gold  and  platinum  will  be  found 
at  the  bottom  of  the  digester.  The  liquid  is  now 
poured  off,  and  the  silver  recovered  from  the  sulfate 
solution  by  precipitation  with  plates  of  copper,  which 
reduce  it  in  a  more  or  less  crystalline  state.  The  re- 
maining alloy,  now  consisting  of  gold  and  platinum, 
should  be  thoroughly  washed,  dissolved  in  nitro- hy- 
drochloric acid,  neutralized  by  evaporation,  then  dis- 
solved in  a  large  quantity  of  distilled  water.  It  is  then 
ready  for  precipitation  with  oxalic  acid,  by  which  the 
gold  is  thrown  down,  and  the  platinum  remains  in 
solution  for  subsequent  treatment.  The  gold,  which 
is  now  easily  collected,  should  be  washed,  dried,  and 
heated  to  redness,  when  it  is  ready  for  weighing. 

Lastly,  the  platinum  may  be  recovered  from  the 
solution  by  precipitation  with  ammonic  chlorid. 
When  washed  and  dried  it  will  be  ready  for  weighing, 
and  every  ioo  parts  may  be  considered  as  containing 
44. 28  of  platinum. 

The  composition  of  some  of  the  principal  dental 
amalgam  alloys  now  in  use,  is  shown  in  the  following 
tables  : 


AMALGAMS. 


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DENTAL  METALLURGY. 


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AMALGAMS.  73 

The  following  formula,  with  directions  for  forming 
the  alloy,  kindly  furnished  by  the  late  Dr.  Ambler 
Tees,  was  highly  recommended  by  that  gentleman  as 
affording  excellent  results  : 

Tin .40  dwts. 

Silver 24     " 

Gold 1  dwt. 

Platinum 1     " 

"  The  gold,  silver,  and  platinum  to  be  melted  first 
with  borax  and  kept  in  a  state  of  fusion  for  five  min- 
utes. The  tin  to  be  melted  in  a  separate  crucible,  and 
the  molten  silver,  gold,  and  platinum  to  be  poured 
into  the  fused  tin,  and  the  whole  quickly  poured  into 
a  suitable  ingot-mold  and  reduced  to  powder  with  a 
large  machinist's  file." 


CHAPTER   VI. 

MODES  OF  MELTING  METALS. 

VARIOUS  forms  of  heating  apparatuses,  furnaces, 
etc. ,  are  of  great  importance  to  those  who  con- 
duct metallurgical  operations  on  a  large  scale. 
We  shall,  however,  in  these  pages  confine  ourselves 
exclusively  to  the  needs  of  the  dental  laboratory,*  in- 
cluding a  consideration  of  the  appliances  for  soldering. 
These  operations  are  performed  by  the  use  of  the 
oil  or  alcohol-lamp,  or  the  gas-jet,  or  by  means  of  a 
suitable  stove  or  furnace.  When  kerosene  oil  or  al- 
cohol is  employed,  it  is  of  the  first  importance  to 
select  a  lamp  designed  not  only  to  meet  the  practical 
requirements,  but  also  with  a  view  to  safety.  The 
first  essential  is  to  have  the  wick  large  enough  to 
afford  a  flame  of  sufficient  magnitude  to  enable  the 
operator  to  solder  an  entire  artificial  denture,  or  to 
fuse  from  one  to  two  ounces  of  gold.  This  would  re- 
quire a  wick  one  and  a  quarter  inches  in  diameter, 
and  about  three  inches  long.  Its  connections  with  the 
reservoir  or  body  of  the  lamp  in  which  the  com- 
bustible fluid  is  contained  should  not  be  direct  nor 
in   such    close    proximity  that    explosive   gas  would 

*Full  and  detailed  descriptions  of  the  different  heating  apparatuses, 
together  with  the  most  approved  processes  of  reducing  ores  and  of  melt- 
ing large  quantities  of  metals,  will  be  found  in  Percey's,  Phillips's,  and 
Makins's  works  on  metallurgy. 

74 


MODES   OF    MELTING    METALS. 


75 


be  likely  to  form.  The  Franklin  Safety  Lamp,  a  cut 
of  which  is  annexed  (Fig.  i),  will  be  found  to  answer 
every  requirement.  It  consists  of  a  reservoir  five 
inches  in  diameter  by  two  and  a  half  inches  deep. 
The  wick-holder,  three  inches  long  by  two  and  a  half 
inches  in  diameter,  is  connected  with  the  reservoir 
by  a  curved  tube  five  inches  long  by  three-sixteenths 
of  an  inch  in  diameter.  Thus  a  sufficient  quantity 
of  the  burning-fluid  is  supplied  to  the  wick  to  afford 

Fig.  i. 


a  constant  flame,  while  there  is  no  danger  of  the  heat 
from  the  wick-holder  being  conducted  to  the  reservoir 
to  cause  an  explosion. 

In  cities  and  large  towns  where  gas  is  availabler 
that  agent  is  to  be  preferred,  on  account  of  its  greater 
safety  and  convenience.  A  gas-burner  which  will  be 
found  to  answer  every  requirement  of  the  laboratory 
may  be  constructed  by  attaching  to  the  base  of  an 
ordinary  Bunsen  burner,  such  as  is  sold  at  the  dental 
depots,  a  piece  of  brass  tubing  six  inches  in  length 


76  DENTAL    METALLURGY. 

by  one  and  a  quarter  inches  in  diameter.  Over  the 
top  of  this,  in  order  to  properly  spread  the  flame,  a 
piece  of  fine  brass  wire-gauze  is  fastened  by  means  of 
a  ring  of  sheet-brass,  one-quarter  of  an  inch  in  width. 
Connection  may  be  obtained  with  the  gas-bracket  in 
almost  any  part  of  the  room  by  means  of  flexible 
rubber  tubing.  This  will  be  found  to  answer  all  pur- 
poses of  soldering  as  well  as  of  melting  small  quantities 
of  gold  and  silver. 

The  blast  and  the  flame  are  produced  by  the  blow- 
pipe,* an  instrument  which  has  long  been  used  by 
workers  in  metals  for  the  purpose  of  soldering  together 
small  pieces  of  metal,  and  for  melting  and  reducing 
purposes  generally.  The  ordinary  form  consists  of  a 
conical  brass  tube,  from  two  hundred  to  two  hundred 
and  thirty  or  forty  millimeters  long,  curved  at  the 
narrower  end  to  nearly  a  right  angle,  so  that  the 
flame  may  be  conveniently  directed  upon  the  piece  ot 
metal  to  be  soldered  or  melted,  as  the  case  may  be, 
which  is  held  upon  some  suitable  support,  such  as  a 
piece  of  charcoal,  coke,  or  pumice-stone.  When  the 
blow-pipe  is  used  in  its  simplest  form,  by  the  mouth, 
the  large  end  of  the  instrument  is  held  between  the 
lips,  and  the  small  end  toward  the  flame.  The  blast 
should  not  be  sustained  by  the  respiratory  organs, 
but,  in  order  that  an  unbroken  current  may  be  kept 
up,  the  mouth  should  be  filled  with  air,  to  be  forced 

*The  use  of  the  blow-pipe  for  analytical  purposes  is  credited  to  Anton 
von  Swab,  a  Swedish  Councillor  of  Mines,  who  made  use  of  the  instru- 
ment in  the  discharge  of  his  official  duties  as  early  as  1738.  Since  that 
date  its  use  has  been  widely  extended,  and  the  importance  of  reactions  in 
the  dry  way  produced  under  the  flame  of  the  blow-pipe  is  fully  recognized 
and  established.  For  full  information  on  the  subject  the  student  is  referied 
to  Plattner's  "  Manual  of  Blow-pipe  Analysis." 


MODES    OF    MELTING    METALS.  77 

through  the  blow-pipe  by  the  muscles  of  the  cheeks. 
While  these  are  forcing  the  air  through  the  blow-pipe, 
the  connection  between  the  chest  and  the  cavity  of  the 
mouth  should  be  closed  by  the  palate,  which  thus  per- 
forms the  part  of  a  valve.  The  beginner  is  liable  to 
fall  into  the  error  of  not  closing  the  connection 
between  the  chest  and  the  mouth  at  the  proper 
instant,  and  of  obtaining  the  force  necessary  to  propel 
the  air  through  the  blow-pipe  from  the  lungs.  That 
this  manner  of  using  the  instrument  may  injure  the 
organs  of  respiration  cannot  for  a  moment  be  doubted, 
and  the  operator  should  early  acquire  the  proper 
method,  above  described.  To  avoid  tiring  the 
muscles  of  the  lip  by  continual  blowing,  the  trumpet 

Fig.  2. 


mouth-piece  has  been  recommended  and  is  shown  in 
the  annexed  cut,  Fig.  2.  This  is  merely  pressed 
against  the  open  mouth,  and  an  uninterrupted  blast 
may  be  kept  up  for  a  long  time  without  causing  the 
feast  fatigue  of  the  orbicularis  oris,  since  that  muscle 
takes  but  a  passive  part  in  the  operation.  This  trum- 
pet-piece, however,  should  be  so  curved  as  to 
correspond  with  the  shape  of  the  mouth,  otherwise  it 
will  require  to  be  pressed  very  forcibly  against  the 
lips  in  order  to  prevent  the  escape  of  air. 

The    blow-pipe   should    be    constructed    of  either 
brass  or  German  silver,  as  these  alloys  are  but  poor 


73  DENTAL     METALLURGY. 

conductors  of  heat.  Silver  is  not  well  suited  for  the 
purpose,  because  it  transmits  temperatures  so  readily 
that  it  soon  becomes  too  hot  for  the  fingers. 

A  long-continued  and  steady  flame  maintained  by 
the  mouth  blow-pipe  is  apt  to  cause  disturbances  in 
the  flame  from  the  collection  of  moisture  in  the  tube, 
which  is  liable  to  be  expelled  by  the  pressure  of  the 
air.  To  avoid  this  a  hollow  chamber  is  constructed 
about  midway  in  the  instrument.  The  length  of  the 
instrument  should  be  adapted  to  the  eye  of  the  opera- 
tor, so  that  the  object  upon  which  the  flame  is  directed 
may  be  distinctly  seen. 

An  improvement  in  these  instruments  has  been 
made  by  Mr.  Thomas  Fletcher,  F.C.S.,  of  Warring- 
ton, England  (see  Fig.   3),   by  which   temperatures 

Fig.  3. 


beyond  those  which  can  be  produced  by  the  ordinary 
gas  and  air  blow-pipes  are  attainable.  It  not  only 
gives  temperatures  never  approached  with  the  old 
blow-pipe,  but  it  is  also  in  every  respect  more  con- 
venient, easier  to  use,  and  better  adapted  for  every 
class  of  work.  With  the  same  amount  of  blowing  as 
with  the  common  form,  this  blow-pipe  will  do  nearly 
double  the  work  ;  if  high  temperatures  are  not  re- 
quired, the  labor  of  blowing  is  reduced  in  proportion. 
The  improvement  consists  in  coiling  the  air-tube  into  a 
light  spiral  over  the  point  of  the  jet.  This  coil  takes 
up  the  heat  which  would  otherwise  be  wasted,  and 
utilizes  it  by  heating  the  air  in  its  passage.  The 
author  has  found  this  form  of  mouth  blow-pipe  to  be 


MODES    OF    MELTING     METALS.  79 

well  adapted  for  fine  analytical  operations  by  cupella- 
tion,  as  well  as  for  all  uses  of  the  dental  laboratory. 

Fletcher's  hot-blast  blow-pipe  is  so  constructed 
that  the  air-pipe  is  coiled  around  the  gas-pipe  in  a 
spiral  form,  and  both  are  heated  by  three  small  Bun- 
sen  burners  underneath,  which  are  controlled  by  a 
separate  stop-cock,  as  shown  in  Fig.  4.  It  is  claimed 
that  the  power  of  this  apparatus  is  about  double 
that  of  an  ordinary  blow-pipe  ;  that  when  the  jet  is 
turned   down   to   a   small   point   it   will    readily  fuse 

Fig.  4. 


a  moderately  thick  platinum  wire,  and  that  its  power 
is  nearly  equal  to  the  oxyhydrogen  jet.  This  form  of 
blow-pipe  is  well  adapted  to  continuous-gum  work 
where  the  teeth  are  soldered  to  the  plate  with  pure 
gold.  The  blast  is  obtained  by  means  of  a  foot-blower 
(Fig.  5),  connected  with  the  blow-pipe  (Fig.  4)  by  a 
flexible  rubber  tube.  The  reservoir  of  the  upper  por- 
tion, which  holds  the  air,  is,  when  the  bellows  is  not  in 
operation,  merely  a  disk  of  thick  coffer-dam  rubber, 
which  expands  under  the  pressure  of  the  air  while  the 
bellows  is  in  motion,  and  thus  affords  a  very  compact, 
powerful,  and  effective  arrangement.  The  step  for 
the  foot  is  very  low  and  the  blower  may  be  used  with 


8o 


DENTAL     METALLURGY. 


ease,  whether  the  operator  is  standing  or  seated.     The 
pressure  is  perfectly  steady  and  equal.     If  the  rubber 

Fig.  5. 


disk  is  distended  until  forced  against  the  net,  the  pres- 
sure can  be  increased  to  almost  any  extent  desired. 

Fig.  6. 


It  will  give,  if  required,  a  heavy  and  continuous  blast 
through  a  pipe  of  a  quarter-inch  clear  bore. 


MODES    OF    MELTING    METALS. 


8l 


The  mechanical  blow-pipe  is  not  a  new  idea.  It  has 
been  in  use  by  dentists  for  nearly  fifty  years,  and  in 
extensive   soldering   operations   is   one    of  the  most 

Fig.  7. 


1 


valuable  appliances  of  the  dental  laboratory.  The  in- 
strument as  formerly  made  by  the  late  Mr.  Bishop,  of 
Philadelphia,  is  probably  superior  to  the  more  recent 
forms  (see  Fig.  6). 

The  Burgess  blow-pipe,  illustrated  in  Fig.  7,  is  con- 


82 


DENTAL   METALLURGY. 


structed  on  exactly  the  same  principle  as  the  above, 
and  is  convenient  and  effective. 

The  form  of  oxyhydrogen  blow-pipe  invented  by 
Dr.  J.  Rollo  Knapp,  is  perhaps  the  most  complete 
and   effective   apparatus   for   soldering   and    melting 

Fig.  8. 


the  s.  s.  whits  denta'lmfg.cp. 


operations  in  the  dental  laboratory  that  has  yet  been 
devised.  It  may  be  used  with  equal  facility  in  sol- 
dering the  largest  piece  of  plate-work,  or  the  most 
delicate  crown-work,  and  is  of  particular  value  to 
dentists  who  give  attention  to  continuous-gum  work, 
enabling  them  to  readily  remelt  their  platinum  scraps. 


MODES    OF    MELTING   METALS.  83 

It  is  provided  with  an  iron  stand  in  which  is  secured 
by  a  thumb-screw  a  100-gallon  cylinder  of  nitrous 
oxid  gas.  By  means  of  a  yoke  and  set-screw,  the 
valve  of  the  cylinder  is  connected  with  the  tubes  and 
valves  of  the  blow-pipe  in  such  manner  that  the  pro- 
portions of  a  mixture  of  nitrous  oxid  and  illuminating 
gases  are  under  perfect  regulation  and  control. 

A  cylinder  of  nitrous  oxid  gas  is  placed  in  the  base 
or  stand,  and  fastened  with  the  thumb-screw  A.  The 
yoke  carrying  the  stop-cocks  and  valves  is  attached 
to  the  valve  of  the  cylinder,  and  tightened  with  the 
screw  B.  The  pipe  C  is  connected  by  a  rubber  tube 
to  an  illuminating-gas  bracket.  When  the  apparatus 
is  in  use  the  illuminating-gas  is  turned  on,  and  its  flow 
regulated  by  the  handle  D.  The  handle  G,  over  the 
outlet  H,  is  then  turned,  the  cylinder  valve  is  opened 
by  means  of  the  hand-wheel  I  sufficient  to  permit  the 
escape  of  enough  nitrous  oxid  gas  to  be  detected  by 
touching  the  opening  H  with  the  finger. 

When  the  desired  quantity  of  nitrous  oxid  gas  is 
obtained,  the  flow  is  directed  to  the  mixing  chamber 
and  controlled  by  the  handle  G,  which,  when  in  posi- 
tion, as  shown  in  the  cut,  allows  the  gas  to  pass  freely 
into  the  chamber  K,  where  it  mixes  with  the  illuminat- 
ing-gas. 

Either  or  both  of  the  burners  may  be  used,  and 
the  desired  flame  obtained  by  regulating  the  pressure 
of  the  gases  by  the  handles  controlling  them.  It  is 
an  instrument  of  much  greater  delicacy  than  the 
blow-pipes  commonly  used  by  dentists.  The  flame 
which  it  affords  is  very  small,  but  the  intensity  of 
its  heat  is  such  that  great  care  must  be  exercised  in 
soldering  small  objects  to  prevent  burning  or  even 


84  DENTAL    METALLURGY. 

entire  fusion  of  the  parts  adjacent  to  the  solder.  It  is 
economical  of  time  and  materials,  and  its  perfect 
cleanliness  will  commend  it  to  all  who  work  in  the 
higher  branches  of  mechanical  dentistry. 

When  a  small  quantity  of  gold  or  silver  is  to  be 
melted  by  means  of  the  blow-pipe,  it  is  usually  per- 
formed upon  a  support  formed  of  charcoal.  A  good 
solid  cylindrical  piece  of  thoroughly  charred  pine  coal 
should  be  selected,  and  divided  into  two  equal  halves 
by  a  vertical  cut  with  a  saw.  Upon  the  end  of  one 
half  a  depression  should  be  cut  for  the  reception  of 
the  metal  to  be  melted.  On  the  flat  side  of  the  other 
half,  extending  to  the  end,  the  ingot- mold  should  be 
carved,  of  size  and  shape  governed  by  the  require- 
ments of  the  case.  The  two  halves  should  then  be 
brought  together  and  secured  by  a  piece  of  iron  or 
copper  wire,  when  they  will  be  found  to  practically 
combine  the  requirements  of  crucible  and  ingot-mold. 
The  depression  in  which  the  metal  is  to  be  melted  and 
the  mold  or  receptacle  should  be  connected  by  means 
of  a  gutter  or  groove.  The. flame  is  now  directed 
upon  the  metal,  and  when  thoroughly  fluid  the  char- 
coal is  tilted  so  that  the  fused  metal  will  run  into  the 
mold  prepared  for  it  in  the  opposite  half  of  the  char- 
coal. This  is  probably  the  simplest  form  of  apparatus 
by  which  small  quantities  of  metal  can  be  melted,  and 
is  often  employed  in  the  dental  laboratory  and  by 
jewelers. 

Fletcher  has  devised  an  apparatus  embodying  the 
same  general  principles  as  the  one  just  described,  for 
quickly  obtaining  ingots  of  gold  and  silver  without  the 
use  of  a  furnace.  It  is  shown  in  the  accompanying 
diagram    (Fig.    9)  ;    A,    representing    a    crucible   of 


MODES    OF   MELTING   METALS.  85 

molded  carbon,  supported  in  position  by  an  iron  side- 
plate  ;  C,  the  ingot-mold  ;  D,  clamp,  holding  ingot- 
mold    and    crucible    in    position  ;    B,   cast-iron    stand 
upon  which  the  latter  swivels.     The 
metal    to    be  melted  is  placed  in  the  Fig.  9. 

crucible,  A,  and  the  flame  of  the  blow- 
pipe directed  upon  it  until  it  is  per- 
fectly fused.  The  waste  heat  serves 
to  make  the  ingot-mold  hot.  The 
whole  is  tilted  over  by  means  of  the 
upright  handle  at  the  back  of  the 
mold.  A  sound  ingot  may  be  obtained  by  the  use  of 
this  simple  little  apparatus  in  a  very  few  minutes. 
Simple  contrivances  of  this  kind  are,  however,  not 
applicable  to  melting  operations  involving  quantities 
exceeding  one  ounce.  In  such  cases  it  is  better  to 
employ  a  crucible  and  any  stove  or  furnace  in  which 
the  temperature  can  be  raised  sufficiently.  This  may 
be  accomplished  in  an  ordinary  cooking-  stove,  a  black- 
smith's  forge,  or  a  small  fire-clay  furnace,  by  the  use  of 
anthracite  coal,  coke,  or  charcoal. 

By  far  the  most  convenient,  compact,  and  effective 
furnace  for  melting  from  one  to  ten  ounces  of  gold 
which  has  ever  been  used  is  the  crucible-furnace  (Fig. 
10),  invented  by  Mr.  Fletcher,  which  can  be  obtained 
at  the  dental  depots.  The  furnace  is  perfectly  adapted 
to  the  wants  of  the  mechanical  dentist.  It  is  composed 
of  a  substance  resembling  fire-clay,  but  much  lighter 
in  weight,  and  said  to  possess  only  one-tenth  its  con- 
ducting power  for  heat. 

The  furnace  consists  of  a  simple  pot  for  holding 
the  crucible,  with  a  lid  and  a  blow-pipe,  all  mounted 
on  a  suitable  cast-iron  base.     As  compared  with  the 


86 


DENTAL  METALLURGY. 


ordinary  gas-furnace  it  appears  almost  a  toy,  owing  to 
its  great  simplicity.  The  casing  holds  the  heat  so  per- 
fectly that  the  most  refractory  substances  can  be  fused 
with  ease,  using  a  common  foot-blower.  Half  a  pound 
of  cast  iron  requires  from  seven  to  twelve  minutes  for 
perfect  fusion,  the  time  depending  on  the  gas -supply 
and  the  pressure  of  air  from  the  blower.  The  power 
which  can  be  obtained  is  far  beyond  what  is  required 
for  most  purposes,  and  is  limited  only  by  the  fusibility 

Fig.  io. 


of  the  crucible  and  casing.  The  crucible  will  hold 
about  ten  ounces  of  gold.  An  ordinary  gas  supply- 
pipe  of  T5g-  or  f-inch  diameter  will  work  it  efficiently. 
It  requires  a  much  smaller  supply  of  gas  than  any 
other  furnace  known  ;  about  ten  cubic  feet  per  hour  is 
sufficient  for  most  purposes.  Crucibles  must  not  ex- 
ceed 2^  by  2  inches.  Any  common  blow-pipe  bel- 
lows will  work  the  furnace  satisfactorily,  except  for 
very  high  temperatures  (fusion  of  steel,  etc.),  for 
which  a  very  heavy  pressure  of  air  is  necessary.  In 
size  it  is  but  four  inches  in  diameter  by  three  in  height. 
The  author  has  used  one  in  his   laboratory  for  the 


MODES    OF    MELTING   METALS. 


87 


purpose  of  melting  gold  and  silver  and  for  general 
metallurgical  experiments  for  several  years,  with  the 
greatest  satisfaction  ;  he  has  also  found  it  to  be  most 
admirably  adapted  to  class  demonstration,  for  which 
purpose,  as  a  means  of  illustrating  his  lectures  on 
metallurgy,  he  has  had  frequent  opportunities  to  use  it. 
A  modification  of  the  apparatus  has  been  made, 
adapting  it  to  the  use  of  refined  petroleum  instead  of 
gas  as  a  fuel,  and  thus  rendering  it  of  more  general 
utility  (see  Fig.  11).     Thus  improved,  it  is  said  to  be 

Fig.  11. 


in  no  way  inferior  in  efficiency  to  the  gas-furnace. 
The  burner  of  this  furnace  is  constructed  upon  the 
principle  of  an  atomizer,  which,  of  course,  dispenses 
with  a  wick  ;  it  is  supplied  with  a  device  for  regu- 
lating the  supply  of  oil,  which  is  operated  by  the 
milled  nut  (marked  A)  shown  on  the  top  of  the  reser- 
voir in  the  cut,  and  for  the  supply  of  an  annular  jet  of 
air,  which  is  regulated  by  turning  the  sleeve  (marked 
B).  This  burner  is  so  arranged  that  in  case  any  ob- 
struction should  occur  it  can  be  taken  apart  and 
cleaned  by  separating  the  burner  from  the  reservoir, 
which  is  accomplished  by  loosening  the  small  screws, 


DENTAL  METALLURGY. 


drawing  out  the  oil-tube,  taking  off  the  sleeve  B,  and 
removing  the  inside  tube. 

These  furnaces  are  so  constructed  that  they  may  be 
used  for  either  gas  or  petroleum,  the  lamp  being  fitted 
for  adjustment  in  place  of  the  gas-burner,  so  that  the 
same  apparatus  may  be  used  for  either.  The  blast  is 
obtained  by  means  of  the  foot-blower  shown  on  page 
80,  which  is  connected  with  the  furnace  by  means  of 
India-rubber  tubing,  as  seen  in  Fig.  11. 

Fig.  12. 


AIR  CHECK 


An  injector  gas-furnace  has  also  been  perfected  by 
Mr.  Fletcher,  which  seems  to  be  well  adapted  to  the 
wants  of  the  dentist,  chemist,  or  metallurgist  (see  Fig. 

12). 

The  construction  of  this  apparatus  is  upon  the 
principle  of  the  injector-furnace,  and  it  is  claimed 
that  its  power  and  speed  of  working  are  practically 
without  limit,  depending  only  upon  the  gas-  and  air- 
supply.  With  a  half-inch  gas-pipe  and  the  small  foot- 
blower  (see  page  80)  this  furnace  will  melt  a  crucible 
full  of  cast-iron  scraps  in  ten  minutes.     The  supply 


MODES   OF   MELTING   METALS.  89 

of  gas  required  is  exceedingly  small.  Allowing  five 
cubic  feet  of  gas  for  heating  up,  it  consumes  about 
four  feet  of  gas  for  every  pound  of  cast  iron  melted. 
For  laboratory  purposes  it  is  the  cheapest  and 
most  convenient  furnace  in  use.  It  is  very  simple  in 
construction,  and  consists  of  two  parts, — an  upper 
portion,  which  forms  the  cover,  and  a  lower  part, 
which  holds  the  crucible  while  in  operation. 

Mr.    Fletcher  has  devised  a  gas-lamp   which   has 
given  satisfactory  results  in  melting  zinc  and  lead  for 

Fig.  13. 


dies  and  counter-dies,  and  for  the  tusion  of  all  alloys 
which  may  be  accomplished  in  an  iron  ladle  at  or 
below  a  red  heat  (see  Fig.  13). 

When  gas  is  not  available  the  gasoline  furnaces  used 
by  plumbers  for  melting  solder  have  no  superior  in 
point  of  convenience  and  rapidity.  These  furnaces 
are  made  by  C.  Gefrorer,  of  Philadelphia,  and  are 
shown  in  Fig.  14. 

Crucibles. — The  term  ''crucible"  is  applied  to  a 
chemist's  melting-pot,  made  of  earthenware  or  other 
material,  and  so  called  from  the  superstitious  habit 

7 


go 


DENTAL  METALLURGY. 


of  the  alchemists  of  marking  such  vessels  with  the 
sign  of  the  cross.  The  term  is  now  generally  under- 
stood as  designating  vessels  in  which  metals  are 
melted  in  furnaces  at  high  temperatures.  A  crucible 
should  possess  the  power  of  resisting  high  tempera- 
tures without  fusing  or  softening  It  should  also  be 
capable  of  retaining  sufficient  strength,  when  hot, 
to  prevent  its  crumbling  or  breaking  when  grasped 


Fig.  14. 


by  the  tongs.     Lastly,  it  should  [not  crack  either  in 
heating  or  cooling. 

For  the  purpose  of  melting  metals  crucibles  are 
made  of  clay  with  admixture  of  silica,  burnt  clay, 
graphite,  or  other  infusible  material.  For  the  fusing 
of  platinum,  which  requires  the  intense  heat  of  the 
oxyhydrogen  flame,  they  are  formed  of  lime.  For 
use  in  the  dental  laboratory,  graphite  crucibles, 
which  can  be  obtained  at  the  dental  depots,  will  be 


MODES   OF   MELTING    METALS.  91 

found  to  answer  every  purpose,  and  they  are  thor- 
oughly reliable  in  strength  and  durability.  They 
range  in  size  from  two  to  four  inches  high,  and  are 
specially  adapted  for  use  in  the  Fletcher  gas-furnaces. 
When  the  quantity  of  metal  to  be  melted  is  very  small, 
say  a  half-ounce  of  gold,  the  smallest- sized  Hessian 
crucible  may  be  used  in  the  small  Fletcher  apparatus. 

Crucibles  suitable  for  melting  platinum  or  iridium 
are  formed  of  two  blocks  of  lime,  each  block  having 
a  concavity  or  excavation,  so  that  when  the  two  pieces 
are  placed  together  the  center  is  hollow  ;  it  is  thus 
designed  to  hold  the  scraps  of  platinum  to  be  melted. 
The  lower  block  is  also  arranged  with  a  groove  and 
lip,  so  that  when  the  metal  becomes  fluid  it  may  be 
poured  into  a  suitable  ingot-mold  by  inverting  the 
crucible.  The  compound  flame  is  introduced  by  tubes 
passing  through  the  center  of  the  upper  block  of  lime 
forming  the  cover. 

Before  melting  any  considerable  quantity  of  gold 
the  crucible  should  be  tested,  particularly  if  the 
melting  operation  is  to  be  performed  in  an  ordinary 
coal-stove,  where  a  defective  crucible  might  be  the 
means  of  a  considerable  loss.  A  small  amount  of 
borax  should  be  placed  in  the  vessel,  which  should 
then  be  exposed  to  a  high  temperature.  Should  it 
not  be  perfect,  the  borax  glass  will  run  through  and 
glaze  the  surface  on  the  outside.  If  the  crucible  is 
found  to  be  impervious,  it  should  be  so  inverted  while 
yet  hot  that  the  borax  glass  may  cover  the  surface 
of  the  lip  or  groove  out  of  which  the  melted  metal  is 
to  be  poured.  This  facilitates  the  pouring  and  pre- 
vents any  portion  of  the  metal  from  adhering  to  the 
side  of  the  crucible. 


92 


DENTAL    METALLURGY. 


Ingot-molds  are  constructed  of  various  substances. 
For  the  reception  of  platinum  melted  with  the  oxy- 
hydrogen  blow-pipe  they  are  formed  of  lime  or  coke  ; 
for  gold  and  silver,  they  are  commonly  made  of  cast 
iron,  about  two  inches  square  and  from  an  eighth  to 
three- sixteenths  of  an  inch  thick  (see  Fig.  15),  with 
slightly  concave  inner  surfaces,  as  the  shrinkage  of 
the  ingot   is  greatest   at   the   center.       Ingot- molds 


Fig.  15. 


formed  of  soap-stone  are  also  employed.     The  ingot- 
mold  should  be  heated  before  pouring. 

Rolling  or  laminating  is  accomplished  by  repeat- 
edly passing  the  metallic  ingot  between  cylindrical 
steel  rollers  from  three  to  four  inches  in  width. 
These  are  so  arranged  that,  by  means  of  screws,  they 
are  capable  of  being  brought  closer  together  every 
time  the  gold  is  passed  through  (see  Fig.  16).  The 
proper  degree  of  attenuation  is  determined  by  the 
gauge-plate  (Fig.   17).     Gold  or  silver  is  made  into 


MODES   OF    MELTING    METALS. 


93 


Fig.  16. 


wire  by  means  of  the  draw-plate, — an  oblong  piece  of 
steel  provided  with  a  number  of  gradually  diminish- 
ing holes  enlarged  on  the 
side  where  the  gold  enters. 
The  gold  to  be  drawn 
through  may  be  prepared 
in  a  cylindrical  shape  by 
melting  and  pouring  into 
an  ingot-mold  provided 
with  a  chamber  for  the  pur- 
pose (some  ingot-molds  are' 
so  constructed).  The  end 
of  the  rod  should  be  filed 
so  as  to  readily  enter  the 
draw-plate,  which  must  be  firmly  screwed  in  a  vise. 


Fig.  17. 


The  gold  is  then,  by  means  of  a  pair  of  strong  pliers, 
drawn  through  the  different  holes  of  the  draw-plate 


94  DENTAL   METALLURGY. 

consecutively  until  the  desired  size  is  reached.  At 
the  beginning  of  the  operation  it  will  require  frequent 
annealing. 

Soldering  must  also,  to  a  certain  extent,  be  regarded 
as  coming  under  the  general  head  of  melting  opera- 
tions, since  it  refers  to  the  union  of  two  or  more  pieces 
of  metal  by  means  of  a  more  fusible  alloy.  The  con- 
ditions of  successful  soldering  are  :  i.  Contact  of  the 
two  pieces  to  be  united.  2.  A  clean  metallic  surface 
over  which  the  solder  is  to  flow.  3.  A  freely  flowing 
solder.     4.   Proper  amount  and  distribution  of  heat. 

Contact  of  the  pieces  to  be  united  is  of  the  greatest 
importance.  If,  for  example,  the  object  to  be  soldered 
be  an  artificial  denture,  it  is  an  indispensable  require- 
ment that  the  backings  be  quite  or  very  nearly  in  con- 
tact with  the  plate,  and,  if  gum  teeth  be  used,  that 
each  backing  touch  its  neighbor.  This  is  not  difficult 
to  accomplish,  if  the  teeth  have  been  carefully  and 
accurately  fitted  to  the  plate  and  to  each  other.  If, 
however,  any  defects  of  this  character  are  found  to 
exist  after  the  teeth  have  been  invested,  they  should 
be  remedied  by  filling  such  spaces  or  crevices  with 
small  pieces  of  gold  or  silver,  as  the  case  may  be, 
thus  rendering  the  continuity  of  the  parts  complete. 
By  the  observance  of  this  precaution  much  of  the 
vexation  in  soldering  experienced  by  beginners  may 
be  avoided,  and  when  the  other  conditions  named 
have  been  observed  the  operation  becomes  exceedingly 
simple.  Solder  runs  freely  by  the  force  of  capillary 
attraction  between  two  closely-fitting  surfaces,  just  as 
water  will  be  drawn  against  gravity  between  two  panes 
of  glass  in  close  contact.  In  soldering  artificial  den- 
tures which  have  been  carefully  arranged  with  refer- 


MODES    OF   MELTING   METALS.  95 

ence  to  contact  of  all  the  parts  to  be  united,  the 
author  has  on  several  occasions  completed  the  opera- 
tion of  soldering  perfectly  without  using  the  blow-pipe 
at  all,  by  merely  heating  the  whole  case  to  the  fusing- 
point  of  the  solder,  in  a  charcoal  furnace  with  a  good 
draft.  Thus  it  will  be  seen  that  the  difficulties  of 
soldering  are  mainly  due  to  a  violation  of  one  or  more 
of  the  rules  herein  given. 

Cleanliness  should  always  be  strictly  observed  in 
soldering  operations.  The  parts  to  be  united  should 
present  bright  and  clean  surfaces.  Darkening  or 
oxidation  will  always  occur  when  gold  or  silver,  the 
purity  of  which  has  been  reduced  by  alloying,  is 
heated  to  redness.  A  weak  solution  of  sulfuric  acid 
and  water,  slightly  heated,  will  quickly  remove  dis- 
coloration resulting  from  this  cause  ;  or  the  borax 
employed  as  a  flux  in  soldering  operations  will  effect 
the  same  result,  by  dissolving*  the  oxid  which  forms 
on  the  surface,  while  it  also  protects  from  further  oxi- 
dation by  excluding  the  oxygen  of  the  atmosphere. 
The  surfaces  to  be  soldered  should  be  carefully  pro- 
tected from  any  contact  with  plaster  of  Paris,  as  there 
is  no  substance  used  in  the  dental  laboratory  more 
likely  to  retard  the  union  of  the  parts  and  impair  the 
final  result  than  this.  To  this  end,  all  parts  over 
which  the  solder  is  to  flow  should,  previous  to  their 
investment  in  sand  and  plaster,  be  thoroughly  covered 
with  the  cement  of  resin  and  beeswax  commonly  used 
in  the  dentist's  laboratory  as  a  temporary  fastening  for 
teeth,  clasps,  etc.,  which  are  to  be  united  by  soldering. 

*  Borax,  when  fused  to  a  glass,  has  the  quality  of  dissolving  metallic 
oxids,  and  the  different  colors  imparted  to  the  "bead"  is  one  means  of 
discrimination  in  blow-pipe  analysis. 


96  DENTAL   METALLURGY. 

This  will  effectually  exclude  plaster,  and  it  is  easily  re- 
moved after  the  investment  has  sufficiently  hardened. 

A  solder  to  be  employed  in  dental  mechanism 
should  possess  the  quality  of  flowing  freely,  and  be  as 
high  in  grade  as  the  attainment  of  that  property  will 
permit,  so  that  it  will  sufficiently  resist  the  action  of 
the  fluids  of  the  mouth.  It  should  also  approximate 
as  nearly  as  possible  the  color  of  the  plate  upon 
which  it  is  used.  If  the  first  condition,  referring  to 
the  contact  of  the  plates  to  be  united,  be  observed, 
the  quantity  of  solder  required  to  effect  continuity 
may  be  reduced  to  the  minimum  ;  and  thus  we  shall 
have  the  smallest  possible  portion  of  the  alloy  exposed 
to  the  action  of  the  fluids  of  the  mouth,  while  we  at 
the  same  time  avoid  the  danger  of  fracture  of  the 
teeth  by  the  contraction  in  cooling  of  an  inordinate 
quantity  of  solder.  The  latter,  for  all  purposes  of  the 
dental  laboratory,  should  be  in  the  form  of  plate 
of  say  No.  27  in  thickness,  and  this  should  be  cut 
into  portions  of  sizes  corresponding  to  the  extent  of 
the  parts  to  be  united.  Thus,  upon  each  pin  a  small 
particle  of  solder  should  be  placed,  just  large  enough 
to  cover  it.  A  piece  of  the  same  size  should  also  be 
placed  near  the  top  of  each  joint  where  the  backings 
come  together,  while  a  larger  piece  should  be  placed 
at  the  points  of  union  between  backings  and  plate. 
These  pieces  of  solder  are  made  to  adhere  to  their 
proper  positions  through  the  agency  of  the  borax, 
which  is  used  by  taking  a  lump,  rubbing  it  on  a  piece 
of  ground  glass  with  clean  water  to  a  creamy  consist- 
ence, and  then  applying  to  the  surface  by  means  of  a 
camel' s-hair  pencil. 

The  application  and  management  of  the  heat  in  the 


MODES    OF   MELTING   METALS.  97 

operation  of  soldering  are  matters  requiring  both  care 
and  judgment.  The  temperature  should  at  first  be 
raised  very  gradually,  in  order  that  pieces  of  solder 
may  not  be  thrown  off  or  displaced  by  the  puffing- up 
incident  to  the  calcination  of  the  borax,  or,  in  the  case 
of  an  artificial  denture,  that  the  porcelain  teeth  may 
not  be  fractured  by  a  too  sudden  elevation  of  tem- 
perature. Both  parts  to  be  united  should  be  equally 
heated  ;  therefore  the  heat  should  be  so  applied  in  the 
case  of  an  artificial  denture  as  to  raise  the  teeth  and 
plate  to  an  equal  temperature  ;  otherwise,  should  the 
plate  become  sufficiently  hot  while  the  teeth  remain 
comparatively  cool  (a  condition  likely  to  occur  unless 
the  fuel  has  been  built  up  around  the  outside  of  the 
investment  covering  the  teeth),  the  solder,  when  the 
flame  of  the  blow-pipe  is  directed  upon  it,  will  flow 
upon  and  adhere  to  the  plate.  In  other  words,  it  will 
manifest  a  preference  for  the  hottest  portion.  The 
failure  to  effect  an  equal  distribution  of  heat  prepara- 
tory to  soldering  is  often  the  cause  of  much  vexation 
and  delay.  For  example,  in  the  process  of  uniting  a 
rim  to  a  plate  by  soldering,  the  rim,  being  so  much 
smaller  than  the  plate,  will  be  more  quickly  heated, 
in  which  event  the  solder  will  fuse  and  flow  upon  the 
rim,  and  the  attempt  to  unite  it  to  the  plate  will  not  be 
successful.  But  to  avoid  such  a  result  the  flame  of  the 
blow-pipe  should,  as  a  preliminary  step,  be  directed 
exclusively  upon  the  plate  until  it  has  been  heated  to 
nearly  the  fusing  point  of  the  solder,  when  the  pointed 
blue  flame  may  be  directed  upon  the  latter,  and  union 
of  the  rim  and  plate  can  hardly  fail  to  take  place. 

Supports. — In  melting  small  quantities  of  gold  or 
silver,  or  in  soldering  with  the  blow-pipe  flame,  it  is 


98  DENTAL    METALLURGY. 

necessary  to  perform  these  operations  upon  a  support 
made  of  some  suitable  body,  such  as  charcoal,  coke, 
pumice-stone,  or  asbestos  and  plaster,  charcoal  and 
plaster,  etc. 

Well-burned  charcoal  is  especially  suited  for  both 
purposes,  as  it  helps  to  increase  the  heat,  and,  in  the 
putting  together  of  small  quantities  of  gold  or  silver 
solders,  prevents  oxidation  of  the  base  metals  which 
are  added  to  reduce  the  fusing-point  of  the  alloy  and 
cause  it  to  flow  freely.  Charcoal  made  from  the  light 
woods,  such  as  pine,  is  best,  because  it  is  not  so  likely 
to  throw  sparks  when  the  flame  is  directed  upon  it 
as  are  the  harder  coals,  such  as  that  made  from  oak, 
and,  being  softer,  it  is  much  better  adapted  to  solder- 
ing operations  in  which  it  is  necessary  to  hold  together 
the  pieces  to  be  united  by  means  of  small  nails  or 
tacks  thrust  into  the  support ;  as,  for  instance,  where 
a  rim  is  to  be  soldered  to  a  plate,  the  former  must 
be  brought  in  contact  with  the  latter  upon  the  char- 
coal, and  so  held  during  the  preliminary  soldering, 
which  consists  of  uniting  the  rim  to  the  plate  with 
a  small  piece  of  solder  at  some  one  point ;  after  which 
the  accurate  adjustment  of  the  rim  to  the  plate  for 
the  final  soldering  is  rendered  much  easier. 

A  good  solid  piece  of  charcoal,  sufficiently  large, 
should  be  selected,  and  bound  with  iron  or  copper 
wire,  to  prevent  its  breaking  into  pieces.  It  should 
then  receive  a  coating  of  plaster,  half  an  inch  in 
thickness,  on  all  sides  except  the  one  upon  which 
the  object  to  be  soldered  is  to  rest.  This  adds  to  its 
strength  and  protects  the  fingers  from  being  soiled 
in  handling  it.  Good  charcoal,  suitable  for  use  in  the 
dental  laboratory,  cannot,  however,  always  be  found 


MODES    OF   MELTING    METALS.  99 

when  wanted,  and  it  is  therefore  often  necessary  to 
use  some  other  substance  which  may  be  more  easily 
obtained.  Thus,  those  living  in  large  cities  may  be 
compelled  to  employ  pieces  of  coke  as  supports  in 
soldering.  Next  to  charcoal,  coke  is  most  suitable 
for  that  purpose.  It  is  more  durable  than  charcoal, 
and  when  such  a  support,  composed  of  one  large 
piece,  or  even  several  smaller  pieces,  is  bound  together 
with  wire  and  coated  with  plaster,  it  will  last  a  long 
time.  Large  pieces  of  pumice-stone  also  answer  well 
for  the  purpose  of  holding  small  objects  while  the 
flame  of  the  blow-pipe  is  directed  upon  them. 
Neither  of  these,  however,  is  so  well  adapted  as 
charcoal  for  holders  when  small  quantities  of  metals 
are  to  be  melted,  in  consequence  of  their  greater 
porosity  and  hardness,  which  prevents  the  cutting 
of  suitable  pits  for  the  reception  of  the  metal  to  be 
fused. 

A  very  good  support  for  soldering  purposes  alone 
may  be  formed  by  filling  a  cup  made  of  sheet  iron 
or  copper,  five  inches  in  diameter  by  five  inches  in 
depth,  with  a  mixture  of  asbestos  and  plaster,  or 
plaster  and  finely-broken  charcoal.  The  vessel  should 
be  supplied  with  a  wooden  handle,  fastened  in  the 
bottom  for  convenience  in  handling. 

Plattner's  "Manual  of  Qualitative  and  Quantita- 
tive Analysis  with  the  Blow- pipe,"  page  15,  gives  a 
method  of  artificially  preparing  good  solid  supports 
of  charcoal  which  might  be  found  of  value  in  the 
dental  laboratory.  It  consists  of  mixing  charcoal 
dust  (which  must  not  be  too  finely  ground)  with 
starch  paste.  The  latter  is  prepared  by  combining 
one  part  of  starch  with  six  parts  of  boiling  water. 


IOO  DENTAL   METALLURGY. 

These  are  stirred  in  an  earthen  pot  until  all  the  mea! 
is  converted  into  paste.  This  paste  is  rubbed  in  a 
porcelain  mortar,  with  frequent  additions  of  charcoal 
dust,  until  the  mass  becomes  too  tough  for  further 
admixture,  when  enough  of  the  coal-dust  is  kneaded 
in  with  the  hands  to  render  the  whole  mass  stiff  and 
plastic.  From  this  the  desired  forms  of  blow- pipe 
coals  can  be  made,  allowed  to  dry  gradually  and 
thoroughly,  and  then  heated  to  redness  in  a  covered 
vessel  so  as  to  char  the  starch  paste.  The  charring 
may  be  regarded  as  complete  when  the  evolution  of 
gases  from  the  mass  ceases,  or  when  it  has  been 
heated  to  dull  redness.  Coals  thus  formed  are  of 
the  proper  firmness,  and  ring  like  ordinary  good 
charcoal  when  thrown  on  the  table. 

Blocks  formed  of  graphite  and  fire-clay  are  now 
often  used  as  supports  for  holding  objects  to  be  sol- 
dered, but  they  do  not  answer  the  purpose  well,  in 
consequence  of  not  being  sufficiently  non-conducting, 
and  they  soon  become  so  hot  in  the  operation  of  sol- 
dering that  it  is  impossible  to  hold  one  in  the  hand  for 
any  length  of  time. 

When  the  object  to  be  soldered  is  an  artificial 
denture  containing  a  number  of  teeth,  a  support  that 
will  be  found  to  answer  all  requirements  is  the  hand- 
furnace,  such  as  is  now  furnished  by  the  dental  depots 
(see  Fig.  18).  It  consists  of  a  funnel-shaped  recep- 
tacle of  sheet  iron,  with  a  grate  or  perforated  plate 
near  the  bottom,  and  a  small  door  on  one  side  under- 
neath the  grate  for  the  admission  of  air.  The  upper 
part  of  the  holder  is  surmounted  by  a  cone-shaped 
top  ;  to  the  bottom  is  attached  an  iron  rod,  five  or 
six   inches   long,    terminating   in   a   wooden   handle. 


MODES    OF    MELTING   METALS. 


IOI 


This  apparatus  is  designed  to  serve  both  the  purpose 
of  heating  the  case  and  as  a  support  or  holder  during 
the  soldering.  For  the  first  it  is  not  well  suited, 
being  too  small  to  contain  fuel  enough  to  admit  of  a 
thorough  heating  of  the  case  ;  but  when  the  object 
to  be  soldered  has  been  brought  to  the  proper  tem- 
perature, it  makes  a  capital  holder  for  a  set  of  teeth 


Fig.  18. 


while  the  flame  of  the  blow-pipe  is  being  directed 
upon  it.  The  best  method  of  heating  up  a  case  is 
to  place  it  on  a  gas-oven,  such  as  is  employed  in  the 
dental  laboratory  for  general  use  and  for  heating 
flasks  in  packing  rubber  work,  etc.  A  ring  of  cast 
or  sheet  iron,  six  inches  in  diameter  by  two  inches 
high,  should  then  be  placed  around  it  for  the  purpose 


102  DENTAL    METALLURGY. 

of  holding  the  charcoal,  which,  in  pieces  the  size  of 
a  hen's  egg,  should  be  built  around  the  outside  of  the 
case,  so  that  it  may  be  uniformly  heated.  The  cone 
or  top  of  the  apparatus  just  described  may  now  be 
placed  over  it.  The  gas  is  then  lighted,  but  the  full 
head  should  not  be  turned  on  until  the  moisture  of 
the  investment  has  been  driven  off,  when  it  may  be 
gradually  increased  until  the  case  is  heated  to  red- 
ness. About  thirty  minutes  will  be  required  to  reach 
the  proper  temperature  for  soldering,  when  the  case 
may  be  lifted  from  the  gas-oven  with  suitable  tongs 
and  placed  in  the  hand-furnace.  The  live  coals  used 
in  heating  up  should  also  be  placed  around  the  out- 
side of  the  investment  to  prevent  the  too  rapid  cool- 
ing of  the  piece,  should  any  delay  in  the  soldering 
occur.  When  the  latter  operation  has  been  satis- 
factorily completed,  the  top  may  be  placed  tightly 
on,  and  all  access  of  air  excluded,  in  order  that  the 
case  may  cool  slowly  and  thus  avoid  the  danger  of 
cracking  the  teeth. 


CHAPTER   VII. 

COMBINATIONS   OF   METALS   WITH    NON-METALLIC 
ELEMENTS. 

THE    metals    combine  with   the    non- metallic   ele- 
ments,  to  form  a  new  class  of  bodies  wherein 
none   of  the   distinctive    characteristics    of  the 
constituents  are  discernible.     These  are  the 

Chlorids, 

Bromids, 

Iodids, 

Fluorids, 

Cyanids, 

Oxids, 

Sulfids. 

Metals  also  form  definite  compounds  with  nitrogen, 
phosphorus,  silicon,  boron,  and  carbon. 

Chlorids. — All  metals  combine  with  chlorin,  and 
some  of  them  in  different  proportions,  as  illustrated 
by  the  stannous  and  stannic  chlorids,  the  first  having 
a  formula  of  SnCl.2,  while  the  composition  of  the  latter 
is  SnCl4.  The  capacity  of  the  metals  for  combination 
with  chlorin  is  not  uniform.  The  different  proportions 
are  designated  by  the  following  terms  : 

Monochlorids,  such  as  KC1. 
Dichlorids,  "     BaO,. 

Trichlorids,  "     AuCl3. 

Tetrachlorids,  "     SnCI4. 

103 


104  DENTAL   METALLURGY. 

The  chlorids  may  be  prepared  by  acting  upon  the 
metals  with  nascent  chlorin  developed  by  hydrochloric 
and  nitric  acids.*  Some  chlorids,  on  the  other  hand, 
are  formed  by  bringing  a  current  of  chlorin  gas  in 
contact  with  the  metal.  In  this  way  titanic  chlorid  is 
formed,  the  chlorin  being  passed  over  a  heated  mixture 
of  charcoal  and  titanic  oxid.  Aluminum  and  chromium 
chlorids  may  be  similarly  obtained. 

The  rationale  of  the  action  of  chlorin  upon  metallic 
oxids  is  that  it  drives  out  the  oxygen  and  unites  with 
the  respective  metals  to  form  chlorids.  The  inter- 
change may  take  place  at  ordinary  temperatures,  as 
in  the  case  of  silver  oxid,  but  in  others  an  elevation 
of  temperature  (sometimes  to  red  heat)  is  required. 

Many  metallic  chlorids  are  prepared  by  acting  upon 
the  metals  with  hydrochloric  acid.  Zinc,  cadmium, 
iron,  nickel,  cobalt,  and  tin  dissolve  readily  in  hydro- 
chloric acid,  with  liberation  of  hydrogen.  Sometimes 
a  chlorid  is  obtained  by  substituting  one  metal  for 
another.  In  this  way  stannous  chlorid  is  frequently 
prepared  by  distilling  metallic  tin  with  mercuric 
chlorid,  thus  : 

HgCl2+Sn=  SnCl2+Hg. 

Lastly,  a  chlorid  may  be  prepared  by  dissolving  a 
metallic  oxid,  hydroxid,  or  carbonate  in  hydrochloric 
acid. 

Bromids. — Bromin  unites  directly  with  most  metals, 
and  forms  compounds  analogous  in  composition  and 
general  properties  to  the  chlorids.  Sea-water  and 
many  of  the  saline  springs  contain  native  bromids, 

*  Aqua  regia, — i.e.,  two  volumes  of  hydrochloric  with  one    volume  of 
nitric  acid. 


COMBINATIONS    OF    METALS,    ETC.  105 

and  silver  bromid  occurs  as  a  natural  mineral.  The 
affinity  of  bromin  for  the  metals  is  inferior  to  that  of 
chlorin,  and  the  latter,  with  the  aid  of  heat,  drives 
out  the  bromin  and  converts  the  substances  into 
chlorids. 

Iodids  are  compounds  possessing  properties  analo- 
gous to  those  of  the  chlorids  and  bromids,  and  are 
obtained  by  processes  similar  to  those  which  yield  the 
latter.  With  the  exception  of  those  of  gold,  silver, 
platinum,  and  palladium,  they  are  not  decomposable 
by  heat  alone. 

Fluorids  are  compounds  formed  by  heating  hydro- 
fluoric acid  with  certain  metals,  or  by  the  action  of 
that  acid  on  metallic  oxids.  They  may  also  be  formed 
by  heating  electro-negative  metals — antimony,  for 
example — with  rluorid  of  lead  or  fluorid  of  mercury. 
The  fluorids  are  destitute  of  metallic  luster,  and  most 
of  them  are  easily  fusible,  and  bear  a  close  resemblance 
to  the  chlorids. 

Metallic  oxids  may  be  variously  formed.  Some 
metals,  by  mere  exposure  to  air  while  heated,  lose  their 
metallic  character,  and,  by  combination  with  oxygen, 
assume  a  totally  different  appearance. 

There  are  several  methods  of  forming  oxids  arti- 
ficially, and  some  oxids  are  capable  of  being  converted 
into  others  of  a  higher  degree.  Red  lead,  for  instance, 
is  thus  formed,  the  metal  being  first  heated  without 
allowing  it  to  fuse,  when  a  protoxid  of  a  yellow  color 
is  formed  ;  but  on  further  exposure  to  a  temperature 
°f  S^-S0  C.,  with  free  access  of  air,  additional  oxy- 
gen is  taken  up,  and  the  mass  assumes  a  brilliant  red 
color. 

Oxids  of  metals  are  also  formed  by  heating  a  nitrate 


106  DENTAL   METALLURGY. 

or  carbonate  to  redness,  by  which  means  the  acid  will 
be  evolved  while  the  oxid  remains.  Thus,  a  protoxid 
of  lead  may  be  formed  by  heating  the  white  carbonate 
of  the  metal,  its  color  soon  changing  to  a  lemon-yellow 
as  the  acid  present  is  driven  off. 

Oxids  of  some  of  the  metals — copper  being  an 
example — are  formed  by  first  acting  upon  the  metal 
with  nitric  acid,  and  in  that  way  obtaining  a  nitrate, 
which  is  dried  and  heated  to  dispel  the  acid,  when  the 
oxid  will  remain. 

Again,  if  we  deflagrate  some  of  the  metals  with  a 
body  containing  a  large  proportion  of  oxygen,  we  ob- 
tain their  oxids.  Tin,  lead,  zinc,  etc. ,  are  in  this  way 
removed  from  alloys  in  which  they  enter  as  prominent 
constituents.  For  example,  take,  say,  one  gramme 
of  an  amalgam  alloy,  consisting  of  tin,  silver,  gold, 
and  platinum,  place  it  in  a  crucible  and  melt  with 
borax.  If  crystals  of  potassium  nitrate  are  then 
dropped  into  the  fluid  mass,  the  tin  is  converted  into 
an  oxid,  and  is  dissolved  and  held  by  the  borax  glass. 
If  this  part  of  the  process  is  thoroughly  performed, 
the  remaining  button  will  be  found  to  contain  only  the 
noble  metals,  silver,  gold,  and  platinum,  which  may 
be  easily  separated  and  weighed,  thus  affording  a  very 
simple  method  of  quantitative  analysis  for  ascertaining 
the  proportions  of  amalgam  alloys. 

Metallic  oxids  in  the  form  of  hydrates  are  obtained 
by  treating  an  aqueous  solution  of  a  metallic  salt 
with  an  alkali.  Thus,  the  hydrated  sesquioxid  of 
iron,  commonly  employed  as  an  antidote  in  arsenical 
poisoning,  is  produced  by  adding  ammonia  to  fer- 
rous sulfate.  Zinc  sulfate  or  cupric  sulfate,  by  the  ad- 
dition of  caustic  potassa,  yields  bulky  hydrated  oxids. 


COMBINATIONS    OF   METALS,    ETC.  IO7 

These  in  turn  may  be  converted  into  simple  oxids  by- 
heat. 

Superficial  oxidation  may  occur  gradually  by  mere 
exposure  to  air  at  ordinary  temperatures,  and  the 
action  will  be  accelerated  by  the  presence  of  moisture. 
It  frequently  occurs,  however,  that  metallic  objects 
thus  superficially  oxidized  are  so  protected  by  the 
newly-formed  oxid  from  further  access  of  air  that 
oxidation  can  no  longer  go  on  ;  but  should  the  rusted 
or  tarnished  surface  of  an  iron  or  leaden  object  be  re- 
moved, oxidation  will  again  occur. 

Many  metallic  oxids  are  formed  during  fusion  of 
the  metals.  Lead  and  zinc  are  examples  of  this.  The 
former,  by  continued  exposure  to  a  sufficient  degree 
of  heat,  may  be  entirely  changed  into  an  oxid,  and 
the  latter,  when  carried  to  a  temperature  much  above 
its  fusing-point,  burns  with  a  brilliant  light,  during 
which  the  oxid  is  evolved  in  the  form  of  white  fumes, 
the  incandescence  accompanying  the  combination 
being  an  evidence  of  the  intense  affinity  which  the 
metal  at  an  elevated  temperature  has  for  oxygen. 
Other  of  the  metals  are  thus  combustible.  The 
familiar  experiment  of  converting  iron  into  an  oxid 
by  throwing  a  jet  of  oxygen  gas  upon  a  red-hot  bar 
of  the  metal  is  an  illustration  of  the  fact,  and  many 
metallic  oxids  may  be  thus  formed  by  deflagration. 

There  are,  however,  a  few  noble  metals  possessing 
so  feeble  an  affinity  for  oxygen  that  they  cannot  be 
made  to  combine  directly  with  the  latter  ;  even  when 
the  oxids  of  these  are  obtained  by  chemical  means, 
the  metals  separate  from  the  oxygen  upon  being 
heated  to  redness.*     Gold  and  platinum  are  illustra- 

*  See  "Noble  Metals." 


IOS  DENTAL   METALLURGY. 

tions  of  this  class  of  metals.  The  latter,  which  is 
employed  as  a  base-plate  in  the  "continuous-gum 
process"  and  for  pins  in  artificial  teeth,  is  subjected 
to  the  most  intense  furnace-heat  without  the  slightest 
oxidation  of  surface. 

Many  of  the  metallic  oxids  occur  in  nature,  a  num- 
ber of  the  metals  being  reduced  from  natural  ores, 
which  are  oxids  of  their  respective  metals,  such  as 
iron,  tin,  manganese,  chromium,  etc. 

Sulfids. — The  metals  unite  with  sulfur  and  form  a 
class  of  compounds  which,  in  a  chemical  and  economi- 
cal point  of  view,  are  almost  as  important  as  the  oxids. 
These  were  formerly  termed  sulfurets.  Many  of  them 
are  found  as  natural  ores,  and  are  generally  brittle 
solids  possessing  a  high  metallic  luster,  the  latter 
quality  being  so  marked  in  some  that  they  have  been 
mistaken  for  gold.  Sulfur  combines  with  the  metals 
in  varying  proportions,  and  it  may  be  observed  that 
combination  takes  place  in  proportions  similar  to  the 
oxids,  the  only  exceptions  to  this  analogy  being  the 
alkalies  and  alkaline  earths, — there  being  but  two 
oxids  of  potassium,  sodium,  and  barium,  while  there 
are  no  less  than  five  sulfids  of  these  metals. 

All  the  metallic  sulfids  are  solid  at  ordinary  tem- 
peratures, most  of  them  fuse  at  red  heat,  and  some 
sublime  unchanged.  The  admission  of  air  to  the 
heated  sulfids  is  followed  by  their  decomposition  and 
conversion  into  sulfates,  or,  if  they  are  exposed  to 
higher  and  continued  heat,  into  oxids.  The  sulfids 
are  all  insoluble  in  water,  with  the  exception  of  those 
of  iodin,  potassium,  strontium,  barium,  and  calcium. 
The  metallic  sulfids  may  be  artificially  formed  by  the 
following  processes  :  by  heating  the  metals  or  their 


COMBINATIONS   OF    METALS,    ETC.  IO9 

oxids  with  sulfur  ;*  and  from  the  sulfates  by  heating 
them  with  charcoal,  or  in  a  current  of  hydrogen  by 
passing  a  stream  of  sulfuretted  hydrogen  through  their 
solutions,  or  by  adding  to  them  a  solution  of  an  alka- 
line sulfid. 

Reduction  of  Metallic  Compounds. 

The  term  "  reduction,"  as  used  in  metallurgy,  refers 
to  the  different  methods  of  separating  a  metal  from 
its  natural  ores  or  from  combination  with  any  non- 
metallic  element.  In  some  cases  this  is  effected  by 
heat  alone.  For  example,  the  noble  metals  are  sepa- 
rated from  oxygen  by  merely  heating  to  6oo°  F. 
(=315.5°  C.)  Generally,  however,  the  joint  action  of 
heat  and  reagents  for  which  the  non-metallic  constitu- 
ents of  the  compound  have  greater  affinity  is  required. 

The  inventions  of  Eugene  H.  and  Alfred  H .  Cowles, 
of  Cleveland,  Ohio,  and  of  Graetzel,  near  Bremen, 
in  Germany,  have  proved  to  be  a  most  important 
advance  in  metallurgy.  The  essential  feature  in  the 
improvements  of  these  gentlemen  is  the  application 
of  the  intense  heat  of  a  current  of  electricity  from 
a  dynamo  machine  through  a  conductor  of  great 
resistance  in  the  presence  of  carbon.  Many  of  the 
most  refractory  ores,  which  have  hitherto  resisted  all 
similar  attempts,  may  be  readily  decomposed  in  these 
electrical  furnaces.  By  this  means  aluminum  is  now- 
reduced  from  corundum,  f 

The  metallic  compounds,  whether  natural  or  artifi- 

*  Silver  is  an  example  of  this.  So  great  is  its  affinity  for  sulfur  that  rub- 
ber, the  indurating  agent  of  which  is  sulfur,  cannot  be  vulcanized  in  con- 
tact with  that  metal. 

fSee  chapter  on  "  Aluminum." 


IIO  DENTAL   METALLURGY. 

cial,  are  a  class  of  bodies  formed  of  dissimilar  elements 
held  together  by  the  force  of  chemical  affinity,  and 
which  are  totally  unlike  either  of  their  constituents. 
This  affinity  varies  much  in  different  metals.  Thus, 
gold  possesses  very  feeble  affinities,  and  when  com- 
bined with  chlorin  it  may  be  partially  precipitated  by 
mere  exposure  to  light  or  the  atmosphere.  The  facil- 
ity with  which  it  often  passes  from  one  element  to 
another  may  be  observed  in  the  interesting  process  of 
manufacturing  "shredded  gold,"*  in  which  an  acid 
solution  of  the  trichlorid  is  formed  and  slightly  heated 
in  a  glass  matrass  ;  gum  arabic  or  sugar  dissolved  in 
water  is  then  added,  when  beautiful  web-like  masses 
of  pure  gold  are  seen  to  form  in  the  liquid,  but  unless 
these  are  quickly  removed  by  means  of  a  glass  spoon 
or  dipper,  they  will  almost  instantly  dissolve  and  the 
gold  again  unite  with  the  chlorin.  Lead,  tin,  zinc, 
iron,  and  many  other  metals  evince  stronger  affinities  ; 
hence,  they  are  not  so  readily  reduced,  and  require, 
in  addition  to  heat,  the  presence  of  other  substances, 
such  as  coal,  coke,  charcoal,  etc.  In  other  words,  it 
is  necessary  to  expose  them  in  contact  with  some 
reagent  between  which  and  the  non- metallic  constitu- 
ents of  the  compound  superior  affinity  exists,  so  that 
by  union  of  these  the  metal  may  be  released.  Indeed, 
it  may  be  truly  said  that  all  analytical  operations  for 
the  reduction  of  ores  and  the  discrimination  and  esti- 
mation of  unknown  bodies  are  performed  by  taking 
advantage  of  the  different  degrees  of  chemical  affinity. 
Thus,  lead  which  has  been  overheated  or  subjected  to 
frequent  or  long-continued  meltings  becomes  partially 
oxidized  and  covered  with   an  earthy-looking  mass 

*  "  Lamm's  shredded  gold."    See  "  Preparations  of  Gold." 


COMBINATIONS   OF    METALS,    ETC.  Ill 

consisting  of  semi-oxidized  metal,  formerly  called  the 
"calx."  Further  exposure  to  heat  would  simply 
have  the  effect  of  converting  this  into  an  oxid  of  a 
higher  degree,  but  if  covered  with  finely-broken  char- 
coal, or  other  carbonaceous  substance,*  the  latter  will 
extract  the  oxygen,  carbonic  acid  will  be  formed  and 
evolved,  while  the  metal  will  be  restored  to  a  free 
state. 

Chlorids. — With  the  exception  of  the  chlorids  of 
the  metals  of  the  alkalies  and  earths,  all  metallic 
chlorids  are  decomposed  when  heated  in  a  current  of 
hydrogen,  hydrochloric  acid  and  the  pure  metal  being 
the  result.  The  chlorids  of  gold  and  platinum  are 
decomposed  by  simple  ignition. 

Argentic  chlorid,  when  heated  on  charcoal,  under 
the  flame  of  the  blow-pipe,  yields  pure  silver  and 
emits  an  odor  of  hydrochloric  acid.  Placed  in  water 
acidulated  with  sulfuric  or  hydrochloric  acid,  argentic 
chlorids  may  be  reduced  by  the  addition  of  pieces  of 
some  easily  oxidized  metal,  such  as  zinc  or  iron,  the 
rationale  of  the  reaction  being  as  follows  :  the  zinc 
displaces  the  hydrogen  of  the  H,So4,  zincic  sulfate 
is  formed,  the  liberated  hydrogen  unites  with  the 
chlorin  to  form  hydrochloric  acid,  and  pure  silver 
remains. 

Sulfuric  acid  decomposes  the  chlorids  and  converts 
them  into  oxids,  the  oxygen  being  supplied  from  the 
water  present.  Some  chlorids  may  be  decomposed  by 
heating  them  with  a  metal  which  has  more  powerful 
basic  properties.  Thus,  sodium,  when  heated  with 
aluminum   or  magnesium   chlorid,  will  become  sodic 

*In  the  dental  laboratory  beeswax  is  usually  employed  to  deoxidize  lead 
or  zinc  which  has  become  thick  and  earthy  by  frequent  meltings. 


112  DENTAL   METALLURGY. 

chlorid,  with  liberation  of  the  magnesium  or  aluminum. 
Some  chlorids  are  reduced  by  heating  with  a  mixture 
of  sodic  carbonate  and  charcoal ;  other  carbonaceous 
compounds,  such  as  sodic  or  calcic  carbonate,  are  fre- 
quently used. 

Sul fids. — Reduction  of  the  sulnds,  in  some  few 
instances,  such  as  those  of  gold,  silver,  and  platinum, 
is  effected  by  heat  alone.  The  oxygen  of  the  atmos- 
phere unites  with  the  sulfur,  which  is  evolved  as  sul- 
furous  acid.  In  many  cases,  however,  a  portion  of 
the  oxygen  combines  with  the  metal,  and  an  oxid 
instead  of  the  free  metal  is  obtained.  The  reduction 
of  many  of  this  class  of  ores  consists  simply  in  such 
interchanges.  The  application  of  heat  and  air  in  some 
instances  converts  the  sulfid  into  a  sulfate,  which  in 
turn  may  be  decomposed  at  high  temperatures  and 
separated  into  sulfurous  acid  and  a  metallic  oxid.  On 
the  other  hand,  some  of  the  sulnds  may,  when  heated 
with  access  of  air,  be  converted  into  permanent  sul- 
fates capable  of  resisting  high  degrees  of  heat.  The 
sulnds  of  the  noble  metals,  when  heated,  part  directly 
with  the  whole  of  their  sulfur,  leaving  the  metal  in  a 
pure  state.  Silver  sulfid  thus  reduced  is  also  partially 
oxidized,  so  that  a  small  portion  of  argentic  sulfate  is 
formed,  which  requires  for  its  reduction  a  still  greater 
elevation  of  temperature. 

Reducing  agents,  such  as  metallic  iron,  hydrogen, 
chlorin,  etc  ,  are  frequently  employed  to  combine 
with  sulfur.  If  sulnds  of  lead  be  heated  with  iron, 
sulfid  of  iron  and  metallic  lead  result.  This  method 
is  frequently  practiced  in  the  assay  of  galena,  clean 
iron  nails  being  heated  with  the  ore.  The  sulnds  of 
antimony,  bismuth,  copper,  tin,  and  silver  are  readily 


COMBINATIONS    OF    METALS,    ETC.  113 

reduced  by  passing  dry  hydrogen  over  them  at  red 
heat,  the  result  of  the  reaction  being  the  free  metal 
and  sulfuretted  hydrogen,  the  product  of  the  union 
of  the  hydrogen  and  sulfur.  Dry  chlorin  will  also 
decompose  them,  and  combine  with  both  the  metal 
and  the  sulfur.  Nitro-hydrochloric  acid  converts  the 
sulfids  into  chlorids,  and  hydrochloric  acid  in  a  few 
instances  acts  similarly  ;  its  hydrogen,  combining  with 
the  sulfur,  is  evolved  as  sulfuretted  hydrogen.  Strong 
nitric  acid  also  decomposes  them,  and  is  often  employed 
in  analyses  of  ores.  The  sulfur  being  thus  oxidized, 
the  liberated  metal  combines  with  the  acid  to  form  a 
nitrate,  mercuric  sulfid  or  native  cinnabar  being  the 
only  ore  which  cannot  be  thus  reduced. 

Oxids. — The  reduction  of  lead,  zinc,  or  tin,  the 
working  qualities  of  which  have  been  impaired  by 
frequent  meltings  with  exposure  to  air,  may  be  effected 
in  the  laboratory  by  placing  the  metal  to  be  treated 
either  in  a  large  clay  crucible  or  in  the  ordinary  iron 
melting-pot  employed  by  dentists.  The  semi-oxidized 
metal  is  then  covered  with  powdered  charcoal,  when 
the  reaction  described  above  takes  place,  and  the 
original  properties  of  the  metal  are  restored. 

There  are  some  oxids  to  which  the  foregoing  treat- 
ment is  not  applicable,  but  these  may  be  reduced 
by  passing  a  current  of  dry  hydrogen  over  them 
when  heated  to  redness.  Makins  gives  the  following 
very  clear  description  of  this  method  of  reducing 
oxids  : 

"  A  large  two-necked  bottle  is  fitted  up  in  the  usual 
way  for  the  evolution  of  hydrogen.  This  has  its 
delivery-tube  passed  into  a  tube  filled  with  fragments 
of  calcic   chlorid,  for  the  purpose  of  absorbing  the 


114  DENTAL   METALLURGY. 

moisture  which  may  be  carried  over  with  the  gas  ;  to 
the  other  end  of  this  drying-tube  is  connected  the  tube 
which  is  to  hold  the  metallic  oxid  (generally  in  a  bulb 
blown  upon  this).  The  gas  bottle  should  contain 
about  a  couple  of  quarts,  so  as  to  afford  a  steady  sup- 
ply, and  the  calcic  chlorid  tube  should  be  long  and  well 
filled.  In  operating,  after  the  gas  has  completely 
driven  out  the  air  in  the  apparatus,  heat  is  applied  to 
the  bulb  containing  the  oxid,  and  its  reduction  will  be 
brought  about.  The  gas  must  be  kept  up  in  a  good 
stream,  so  as  to  drive  out  the  watery  vapor  formed  by 
the  decomposition.  Here  the  hydrogen  takes  the 
oxygen  of  the  oxid,  and  water  is  formed,  while  the 
metal  is  set  free. ' ' 

There  are  metals  whose  affinity  for  oxygen  is  so 
strong  that  their  union  with  that  element  cannot  be 
broken  up  by  such  means  as  we  have  described. 
Deoxidation  of  these  metals  must  be  performed 
through  the  agency  of  some  other  metal  possessing 
greater  affinity  for  oxygen.  For  example,  if  oxid  of 
iron  be  heated  with  potassium,  the  iron  will  be  de- 
oxidized, while  the  potassium  will  be  converted  into 
potash  (K20). 

Some  metallic  oxids  may  be  reduced  by  heating 
with  sulfur,  part  of  the  latter  abstracting  the  oxygen r 
with  which  it  unites  to  form  sulfurous  acid.  A  portion 
of  the  sulfur,  however,  unites  with  the  metal,  which  is 
converted  into  a  sulfid,  or  a  sulfate,  or  a  mixture  of 
both.  These  must  then  be  treated  according  to  the 
directions  already  given  for  the  reduction  of  metals 
when  combined  with  sulfur. 

There  are  also  a  few  metallic  oxids  which  chlorin 
gas   will  reduce.     Thus,    platinum  is  liberated  from 


COMBINATIONS    OF    METALS,    ETC.  115 

combination  with  oxygen  when  exposed  to  a  current 
of  dry  chlorin. 

Probably  the  most  powerful  means  of  reducing 
metals  from  combination  with  non-metallic  elements 
is  that  known  as  electrolysis.  It  consists  in  exposing 
a  solution  of  a  metallic  salt  to  the  decomposing  in- 
fluence of  the  galvanic  current.  A  demonstration  of 
this  force  may  be  made  by  taking  a  solution  of  nitrate 
of  lead  (plumbic  nitrate)  and  immersing  in  it  a  piece 
of  zinc.  The  latter  soon  becomes  covered  with  needle- 
like crystals  of  pure  lead  ;  the  zinc  replaces  the  lead, 
which  is  set  free  and  deposited  at  the  point  of  galvanic 
action.  Or,  the  same  phenomenon  may  be  witnessed 
by  immersing  a  piece  of  clean  iron  in  a  solution  of 
copper,  or  a  piece  of  copper  in  a  solution  of  a  salt  of 
mercury,*  the  action  only  ceasing  when  all  the  metal 
in  the  solution  is  reduced. 

*  Reinsch's  test  for  the  detection  of  the  mineral  poisons  is  based  upon 
this  principle. 


CHAPTER  VIII. 

GOLD. 
Atomic  Weight,  197.    Symbol,  Au  (Aurum). 

GOLD  is  one  of  the  few  metals  which  is  found  in 
the  metallic  state,  and  it  was  probably  one  of  the 
first  known  to  man.  Allusions  to  it  are  frequent 
in  the  Old  Testament,  and  jewelry  and  vessels  found 
in  Egyptian  tombs  afford  evidence  of  the  perfection 
attained  in  working  it  at  a  period  earlier  than  the 
government  of  Joseph.  There  are  many  evidences 
that  processes  of  alloying,  refining,  and  separating 
gold  were  practiced  at  a  very  early  period  of  the 
world's  history.  According  to  Pliny,  the  metallurgy 
of  gold  was  known  in  his  day.  Vitruvius  also  gives 
a  detailed  account  of  the  method  of  recovering  gold 
by  amalgamation  from  cloth  into  which  it  had  been 
woven.  It  was  employed  in  Rome  for  the  purpose 
of  fixing  artificial  teeth  more  than  three  hundred 
years  before  the  Christian  era,  and  a  law  of  the 
' '  Twelve  Tables' '  makes  exception  with  regard  to 
such  gold,  permitting  it  to  be  buried  with  the  dead.* 
The  great  beauty  of  color  and  luster,  and  the  power 
of  resisting  oxidation  which  gold  possesses,  have 
caused  it  to  be  valued  from  the  earliest  ages  for  the 
purpose  of  adornment,  and  as  a  circulating  medium. 

*  Phillips's  Metallurgy. 
Il6 


GOLD.  Iiy 

Occurrence,  Distribution,  and  Properties. — Gold  is 
of  nearly  universal  distribution,  and  is  found  in  nature 
chiefly  in  the  metallic  state  as  native  gold.  It  occa- 
sionally occurs  in  combination  with  tellurium,  lead, 
and  silver,  forming  a  peculiar  group  of  minerals,  con- 
lined  to  a  few  localities  in  Europe  and  America,  these 
being  the  only  certain  examples  of  natural  combina- 
tions of  the  metal.  The  most  important  minerals 
containing  gold  are  sylvanite  or  graphic  tellurium, 
(AgAu)Te,,  containing  about  twenty-four  per  cent, 
of  gold  ;  calaverite,  AuTe2,  containing  about  forty 
per  cent,  of  gold,  and  nagyagite  or  foliate  tellurium, 
the  composition  of  which  is  not  definitely  known.  It 
contains  from  five  to  nine  per  cent,  of  gold.  The 
metallic  sulrids,  such  as  galena  and  iron  pyrites, 
usually  contain  sensible  quantities  of  gold,  the  lead 
ore  being  almost  invariably  gold-bearing.  Native 
arsenic  and  antimony  also  occasionally  contain  gold, 
and  a  native  gold  amalgam  has  been  found  in  Cali- 
fornia. 

Gold  occurs  in  nature  very  nearly  though  never 
quite  pure,  being  generally  associated  with  silver. 
Other  metals  are  occasionally  found  combined  with  it, 
but  in  very  small  quantities  ;  and  these  foreign  metals 
are  peculiar  to  localities.  Thus,  California  gold,  in 
addition  to  silver,  which  is  always  present,  may  contain 
iridium  ;  Russian  gold  often  contains  platinum,  and 
specimens  of  the  native  metal  from  Brazil  will  not  in- 
frequently be  found  to  contain  palladium. 


n8 


DENTAL  METALLURGY. 


Analyses  of  Native  Gold  from  Various  Localities. 


Gold. 

Silver. 

Iron. 

Copper. 

United  States  :* 

California  .... 

90.12 

9.OI 

6.15 

.    .   . 

Europe : 

Vigra  &  Clogau  .     . 

90.16 

9.26 

trace. 

trace. 

Wicklow  (river)  .     . 

92.32 

6.17 

0.78 

Transylvania  .    .     . 

60.49 

38.74 

.  .  . 

0.77 

Asia  : 

Russian  Empire — 

Brezovsk    .... 

91.81 

8.03 

trace. 

0.09 

Ekaterinburg      .     . 

98.96 

O.16 

0.05 

o.35 

Africa  : 

Ashantee   .... 

90.05 

9-94 

.  .  . 

.  .  . 

America  : 

94.0 

5-85 

.  .  . 

.  .  . 

Central  America .     . 

88.5 

11.96 

.  .  . 

.  .  . 

76.41 

23.12 

.  .  . 

0.87 

Cariboo      .... 

84.25 

14.90 

.  .  . 

0.03 

Australia  : 

South  Australia .     . 

87.78 

6.07 

6.15 

.  .  . 

Ballarat.     .... 

99-25 

0.65 

.  .  . 

.  .  . 

Pure  gold  is  of  a  rich  yellow  color,  and  is  nearly  as 
soft  as  lead.  It  is,  with  one  exception  (platinum), 
the  heaviest  substance  in  nature,  being  about  nine- 
teen and  a  half  times  as  heavy  as  water.  These  pro- 
perties are  all  sensibly  modified  by  admixture  of  other 
metals.     Thus,  the  tint  is  lowered  by  small  quantities 

*  The  yield  of  gold  in  the  United  States  in  1876  was  greatly  in  excess 
of  that  of  any  other  countr/  on  the  globe,  Russia  being  next  in  quantity 
produced. 


GOLD.  II9 

of  silver,  and  heightened  by  copper.  Owing  to  its 
exceeding  softness,  gold  is  commonly  used  alloyed,  in 
order  to  render  it  capable  of  resisting  the  attrition  to 
which  coins  and  articles  of  jewelry  are  exposed.  It  is 
the  most  malleable  of  all  the  metals.  One  grain  may  be 
beaten  into  leaves  which  would  cover  a  surface  of  fifty- 
six  square  inches,  and  only  -g-tf-oVoo"  of  an  inch  thick.* 

Very  thin  gold  leaf  appears  yellow  by  reflected  and 
green  by  transmitted  light.  Highly  attenuated  films 
of  gold,  when  heated,  transmit  rays  of  light  of  a  ruby- 
red  color.  The  pressure  of  a  hard  substance  on  the 
film  will,  however,  so  change  its  state  of  aggregation 
that  the  green  color  will  again  appear. 

Gold  is  exceedingly  ductile,  but  does  not  possess  a 
very  considerable  degree  of  tenacity.  A  grain  of 
gold,  however,  if  covered  by  a  more  tenacious  metal, 
such  as  silver,  may  be  drawn  into  a  wire  five  hundred 
feet  in  length.  It  also  possesses  the  remarkable  pro- 
perty of  welding  cold.  Thus,  the  metal,  in  the  state 
in  which  it  is  obtained  by  precipitation  by  oxalic  acid, 
may  be  formed  into  disks  or  medals  by  compression 
between  dies. 

The  specific  gravity  of  gold  varies  according  to 
condition.  In  the  finely  divided  state  in  which  it  is 
obtained  by  precipitation  by  oxalic  acid  it  is  19.36. 
The  specific  gravity  of  cast  gold  is  somewhat  less, 
but  when  compressed  between  dies,  or  by  the  rolling- 
mill,  it  may  be  raised  from  19-37  to  T9-4X-  Annealing, 
however,  will  restore  its  previous  density  to  nearly 
that  of  the  cast  metal. 

The  late  Dr.  S.  S.  White  presented  the  author  with  a  specimen, 
securely  mounted  between  plates  of  glass,  which  is  but  3"otfV<J<X  of  an  inch 
in  thickness.     It  is  transparent,  and  transmits  green  rays  of  light 


120  DENTAL   METALLURGY. 

The  atomic  weight  of  gold  has  been  variously- 
stated.  Berzelius  gave  it  as  196.67  ;  Levol,  196.3  ; 
Wurtz,  196.5  ;  Watts,  196.0;  Bloxam,  196.6  ;  Fownes, 
197.  There  seems  also  to  be  a  similar  diversity  of 
opinion  regarding  the  temperature  at  which  it  fuses. 
Thus,  Daniell  fixed  the  melting-point  at  14250  C. ; 
Pouillet,  12000  C.  ;  Guy  ton  de  Morveau,  13800  C. 
The  figures,  no2°C,  given  in  Fownes's  "Elementary 
Chemistry,"  are  probably  as  nearly  correct  as  any, 
and  for  all  practical  purposes  will  answer  very  well. 

The  electric  conductivity  of  gold  is  given  by  Mat- 
thiesen  as  73.96  at  15.  i°  C,  pure  silver  being  100. 
The  conducting  power,  however,  depends  much  upon 
the  degree  of  purity,  as  the  smallest  addition  of  another 
metal  will  very  considerably  lower  its  conductivity.* 

The  conductivity  of  gold  for  heat  is  stated  as  53.2, 
as  compared  with  pure  silver,  100.  Its  specific  heat 
is  0.0324. 

Volatility. — The  absence  of  uniformity  in  results  of 
experiments  with  regard  to  this  property  given  by 
different  investigators  would  seem  to  leave  the  matter 
still  in  doubt.  Thus,  Gasto  Claveus  and  Kunkel 
describe  similar  experiments,  wherein  an  ounce  of 
pure  gold  was  placed  "in  an  earthen  vessel  in  that 
part  of  a  glass-house  where  the  glass  is  kept  con- 
stantly melted,  and  retained  in  a  state  of  fusion  for 
two  months,  without  the  loss  of  the  smallest  portion 
of  its  weight."  On  the  other  hand,  Homburg,  La- 
voisier, and  Maquer  state  that  when  a  small  portion 
of  gold  is  kept  at  a  violent  heat  part  of  it  is  volatil- 
ized, and  that  a  piece  of  silver  held  in  the  rising 
fumes  will  have  its  surface  gilded.     It  is  quite  prob- 

*  See  "  Power  of  Conducting  Electricity." 


GOLD.  121 

able  that,  when  a  small  portion  of  gold  is  mixed  with 
a  large  quantity  of  zinc  and  heated  in  the  air,  the 
whole  of  the  gold  will  be  dissipated  with  the  fumes 
of  oxid  of  zinc.  Mr.  Makins  has  demonstrated  that 
gold,  silver,  and  lead,  when  cupelled  together,  volatil- 
ize. Gold  may  also  be  volatilized,  when  in  the  form 
of  leaf  or  highly-attenuated  wire,  by  passing  a  power- 
ful charge  of  electricity  through  it. 

Forms  of  Native  Gold. — The  native  metal  is  some- 
times found  in  the  form  of  cubic  crystals,  in  octahe- 
dra,  and  in  irregular  and  more  complex  shapes  called 
nuggets  and  dust.  Crystals  of  gold  may  also  be 
obtained  artificially  from  an  amalgam  of  gold  one 
part,  mercury  twenty  parts.  The  mixture  is  rriain- 
tained  at  a  temperature  of  8o°  C.  for  eight  days. 
The  mercury  is  then  removed  by  strong  nitric  acid, 
leaving  crystals  of  gold,  which  require  to  be  heated 
to  redness  to  develop  brilliancy  of  surface. 

Gold  is  found  in  quartz  veins  or  reefs  traversing 
slaty  or  crystalline  rocks,  alone  or  associated  with 
iron,  copper,  magnetic  and  arsenical  pyrites,  galena, 
specular  iron  ore,  and  silver  ores,  and  more  rarely 
with  sulfid  of  molybdenum,  tungstate  of  calcium, 
bismuth,  and  tellurium  minerals.  It  is  also  found 
among  the  detritus  of  disintegrated  rock,*  associated 
with  the  metals  of  the  platinum  group.  In  the  super- 
ficial alluvial  or  "placer"  deposits  it  has  been 
remarked  that  the  minerals  with  which  it  is  found 
intermixed  are  of  great  density  and  hardness,  and  are 
the  most  durable  constituents  of  disintegrated  rock. 

The  yield  of  gold  in  easily-worked  alluvial  deposits 

*  In  the  form  of  small  lumps  called  "dust." 
9 


122  DENTAL    METALLURGY. 

is  often  exceedingly  small.  It  is  stated  that  in  the 
Siberian  gold-washings  the  proportion  of  gold  ranges 
from  12  grains  to  i  dwt.  to  the  ton  of  sand,  while  in 
the  lodes  which  require  more  labor  to  work  the  pro- 
portion is  but  8  dwts.  per  ton,  and  in  the  "placer" 
washings  of  California  it  is  but  12  grains  to  the  ton  of 
gravel.  In  Australia  the  alluvial  washings  of  Victoria 
yielded  25  grains  to  the  ton.  Vein  mining  being  more 
difficult  and  costly,  necessitates  a  larger  yield  of  the 
precious  metal,  and  5  dwts.,  or  about  five  dollars' 
worth  of  the  gold,  is  in  most  gold-bearing  localities 
regarded  as  a  paying  quantity. 

The  method  of  obtaining  gold  from  alluvial  deposits 
is  exceedingly  simple,  and  consists  in  washing  away 
the  lighter  portions,  leaving  the  heavy  metallic  parti- 
cles. In  the  early  days  of  gold-mining  in  California 
this  was  accomplished  by  means  of  a  pan  of  sheet 
iron,  thirteen  or  fourteen  inches  in  diameter,  held  in 
the  hand  and  its  contents  exposed  to  a  stream  of 
water  ;  and  on  the  large  scale  consists  of  washing  the 
alluvial  deposits  into  sluices  or  troughs  by  means  of 
continuous  streams  of  water, — mercury  or  amalgam- 
ated copper  plates  being  sometimes  employed  to 
collect  the  finer  particles  of  gold. 

In  vein  mining  the  separation  of  the  gold  from  the 
rock  with  which  it  is  mechanically  mixed  consists  in 
reducing  the  latter  to  a  fine  powder  in  grinding-  or 
stamping-mills,  and  the  gold  is  recovered  by  amalga- 
mation, or  by  washing  the  pulverulent  mass  through 
troughs  lined  with  coarse  woolen  cloths,  by  which 
means  the  lighter  deposits  are  carried  away  with  the 
current,  while  the  heavier  metallic  particles  become 
entangled  in  the  fibers  of  the  blanket  until  the  surface 


GOLD.  123 

of  the  latter  is  completely  covered,  when  it  is  removed 
and  its  contents  are  washed  off  in  a  suitable  vessel  and 
reserved  for  amalgamation. 

In  the  treatment  of  gold  by  amalgamation  the  pro- 
cess is  frequently  retarded  by  a  difficulty  known  as 
the  "sickening"  or  "flouring"  of  the  mercury. 
The  latter,  losing  its  bright  metallic  surface,  is  no 
longer  capable  of  coalescing  with  other  metals.  The 
discovery  was  made  by  Wurtz,  in  1864,  that  by  the 
addition  of  a  small  quantity  of  sodium  to  the  mercury 
the  operation  is  greatly  facilitated,  the  addition  of  the 
sodium  preventing  both  the  conditions  above  referred 
to  which  are  produced  by  certain  associated  minerals. 
Some  metallurgists  recommend  the  addition  of  20  per 
cent,  of  zinc  and  10  per  cent,  of  tin.  It  has  been 
estimated  that  mercury  will  dissolve  from  0.05  to  0.08 
per  cent,  of  native  gold  of  standard  650  to  850  with- 
out loss  of  fluidity.  The  solubility  of  the  gold  in- 
creases with  its  fineness.  When  the  point  of  satura- 
tion has  been  reached,  lumps  of  the  solid  amalgam 
are  introduced  into  an  iron  vessel  lined  with  a  mixture 
of  fire-clay  and  wood-ashes,  and  provided  with  an 
iron  tube,  by  which  the  fumes  of  mercury  are  passed 
through  water  and  condensed,  the  distillation  being 
effected  at  a  temperature  below  redness.  The  gold 
left  in  the  retort  is  then  melted  in  a  suitable  crucible. 

Gold  is  sometimes  reduced  from  the  mineral  by 
exposing  the  ore,  which  has  been  previously  roasted, 
to  a  current  of  chlorin  gas.  By  this  means  the  gold 
is  converted  into  a  soluble  chlorid,  which  is  removed 
by  washing  with  water.  The  precious  metal  is  then 
recovered  in  the  metallic  form  by  precipitation  with 
ferrous  sulfate.     This  process  is,  when  carefully  per- 


124  DENTAL   METALLURGY. 

formed,  a  very  accurate  one,  and  yields  97  per  cent. 
of  the  gold  present  in  the  ore. 

Refining  Gold. — Methods  of  refining  gold  were 
known  and  practiced  in  very  ancient  times,  and  many 
of  them,  though  empirically  employed,  did  not  ma- 
terially differ  in  principle  from  those  in  use  at  the 
present  day.  Thus,  in  Strabo's  time  the  gold  was 
placed  on  the  fire  with  three  times  its  weight  of  salt 
and  a  quantity  of  argillaceous  rock,  which  in  the 
presence  of  moisture  effected  the  decomposition  of 
the  salt.  Hydrochloric  acid  is  thus  formed,  which, 
at  the  high  temperature  employed,  furnishes  chlorin 
to  the  silver  associated  with  the  gold,  which  is  con- 
verted into  a  chlorid.  A  similar  process  is  still  prac- 
ticed in  South  America. 

Among  other  methods  for  the  separation  of  gold 
from  silver  or  other  contaminating  metals,  which  have 
been  in  use  from  a  remote  period',  may  be  mentioned 
prolonged  oxidation  by  exposure  to  air,  and  melting 
with  sulfur,  sulfid  of  antimony,  and  corrosive  subli- 
mate. 

The  old  "  quartation"  process  of  refining,  so  called 
from  the  fact  that  an  alloy  is  formed,  four  parts  of 
which  contain  three  parts  of  silver  and  one  of  gold, 
consists  in  first  forming  an  alloy  of  the  gold  with  sil- 
ver in  the  proportions  given.  These  are  melted  to 
insure  homogeneity,  and  granulated*  by  pouring  into 
water  contained  in  a  wooden  vessel.  The  pieces  are 
then  collected  and  placed  in  a  glass  or  platinum  vessel, 
and  acted  upon  by  either  nitric  or  sulfuric  acid. 
When  the  presence  of  lead  or  tin  is  suspected,  these 

*  Granulation  is  usually  repeated  twice  or  thrice. 


GOLD.  125 

should  be  got  rid  of  before  subjecting  the  alloy  to 
the  acid  ;  otherwise  the  platinum  digester  would  be 
injured.  The  removal  of  lead  is  accomplished  by- 
cupelling  the  alloy,  while  tin  may  be  effectually 
removed  by  fusing  the  alloy  with  potassium  nitrate. 
On  account  of  greater  economy  and  the  closeness 
with  which  it  will  act  upon  silver  containing  very 
small  quantities  of  gold,  sulfuric  acid  is  at  the  present 
day  the  agent  most  commonly  employed,  particularly 
where  large  quantities  of  the  alloy  are  to  be  treated. 

The  nitric  acid  plan  does  not,  as  a  rule,  yield  gold 
of  as  high  a  degree  of  fineness  as  the  sulfuric  acid 
treatment,  but  the  oxidizing  property  of  the  nitric 
acid  is  of  great  advantage  in  refining  gold  contam- 
inated with  antimony  and  other  equally  injurious 
metals. 

When  nitric  acid  is  employed,  each  ounce  of  the 
granulated  alloy  is  treated  with  an  ounce  and  a  quar- 
ter of  nitric  acid  of  specific  gravity  1-32. 

The  sulfuric  acid  process  is  based  upon  the  facts 
that  the  concentrated  hot  acid  converts  silver  and 
copper  into  soluble  sulfates  without  attacking  the  gold, 
the  metallic  silver  being  recovered  from  the  sulfate  in 
the  form  of  needle-like  crystals  by  thrusting  copper 
plates*  into  it.  The  sulfate  of  copper  resulting  from 
the  reaction  is  crystallized  and  becomes  an  article  of 
commerce.  The  sulfuric  acid  should  be  of  specific 
gravity  1-84,  and  the  alloy  is  boiled  for  three  or  four 
hours  in  a  platinum  vessel  with  2-5  times  its  weight  of 
acid.  The  sulfurous  acid  fumes  which  arise  are  par- 
tially condensed  before  being  allowed  to  pass  into  the 
air.     When  the  acid  has  ceased  to  act  upon  the  metal, 

*  Iron  plates  are  sometimes  used. 


126  DENTAL    METALLURGY. 

a  small  quantity  of  sulfuric  acid  of  specific  gravity 
1.53  is  added,  and  after  a  second  boiling  the  contents 
of  the  vessel  are  allowed  to  settle  ;  the  liquid  is  with- 
drawn from  the  gold,  which  rests  at  the  bottom  of  the 
vessel,  and  is  diluted  until  its  density  is  1.21  to  1-26. 
The  gold  is  then  carefully  washed  and  melted  into 
ingots,  which  generally  contain  from  997  to  998  parts 
of  gold  in  the  1000. 

In  the  nitric  acid  process  the  supernatant  liquid 
consists  mainly  of  argentic  nitrate.  The  silver  may 
be  recovered  by  precipitation  with  chlorid  of  sodium, 
argentic  chlorid  resulting,  which  in  turn  is  exposed  to 
a  current  of  hydrogen,  liberating  metallic  silver.* 

The  dry  method  of  refining  gold  before  alluded  to 
consists  in  placing  the  granulated  alloy  and  a  mixture 
of  one  part  of  chlorid  of  sodium  and  two  parts  of 
brick-dust  in  alternate  layers  in  a  crucible  until  the 
latter  is  full,  when  it  is  covered  and  placed  in  a  wood 
fire  and  kept  at  a  dull  redness  for  twenty-four  hours. 
By  the  united  action  of  the  moisture  furnished  by  the 
wood  and  the  silica  of  the  brick-dust  the  sodic  chlorid 
is  decomposed  ;  its  sodium  combines  with  oxygen 
from  the  decomposition  of  the  water,  forming  soda, 
which  in  turn  unites  with  silica  to  form  sodic  silicate. 
The  hydrogen  of  the  water  and  the  liberated  chlorin 
form  hydrochloric  acid  ;  these,  at  the  temperature  at 
which  the  operation  must  be  carried  on,  furnish  chlorin 
to  the  silver,  converting  it  into  argentic  chlorid.  The 
latter,  being  fusible,  is  absorbed  by  the  brick- dust, 
permitting  the  alloy  to  be  further  acted  upon,  until 
nearly  all  the  silver  is  converted  into  chlorid,  the  gold 
remaining  comparatively  free. 

*  See  chapter  on  "  Silver." 


GOLD.  127 

There  is  also  another  cementation  process  given, * 
for  the  purpose  of  acting  upon  the  surface  of  gold 
containing  a  large  percentage  of  silver,  by  which 
means  it  is  made  to  resemble  fine  gold.  It  consists  in 
subjecting  the  alloy,  previously  rolled  thin  and  covered 
by  the  cement-powder,  to  a  temperature  slightly  below 
its  melting-point.  In  this  operation  the  mixture  is 
composed  of  one  part  of  sodic  chlorid,  one  part  of 
alum,  one  part  of  ferrous  sulfate,  and  three  of  brick- 
dust.  At  the  high  temperature  necessary  the  sulfates 
are  decomposed,  with  liberation  of  free  sulfuric  acid, 
while  chlorin  is  evolved  from  the  sodic  chlorid.  These 
act  upon  the  silver,  which  is  subsequently  found  in  the 
cement-powder  in  the  form  of  argentic  chlorid. 

Refining  by  chlorin  gas,  devised  by  F.  B.  Miller, 
of  Sydney,  N.  S.  W.,  in  1867,  is  a  valuable  and 
accurate  dry  method  for  separating  silver  from  gold. 
The  process  is  the  one  now  practiced  in  the  Australian 
mints,  where  it  has  been  quite  extensively  employed, 
1,100,000  ounces  of  gold  having  been  refined  by  it 
in  Sydney  during  one  year,  the  percentage  of  loss 
during  the  operation  being  only  14  parts  in  the 
100,000.  It  consists  in  converting  the  silver  into 
chlorid  by  the  passage  of  a  stream  of  chlorin  gas 
through  the  molten  alloy.  By  means  of  a  clay  pipe 
passing  through  the  cover  to  the  bottom  of  the  cruci- 
ble, and  connected  with  the  chlorin-generator  by 
means  of  a  flexible  tube,  the  gas  is  passed  rapidly 
through  the  melted  metal,  and  is  apparently  absorbed 
by  it.  The  refining  is  considered  as  complete  when 
orange-colored  fumes  begin  to  rise.     As  soon  as  this 

*  This  process  is  attributed  to  Kerl,  and  is  described  in  Makins's  Metal- 
lurgy, p.  224. 


128  DENTAL     METALLURGY. 

evolution  of  gas  is  noticed  the  crucible  should  be 
removed  from  the  fire,  to  prevent  the  gold  itself  from 
combining  with  the  chlorin.  The  chlorid  of  silver, 
which  is  fusible,  should  be  poured  off  from  the  surface 
of  the  molten  metal,  and,  if  it  retains  a  small  portion 
of  the  gold,  this  may  be  recovered  by  fusing  with  a 
little  carbonate  of  soda,  which  causes  the  gold  to 
separate  and  settle  to  the  bottom  of  the  crucible. 
This  method  is  capable  of  producing  gold  of  from  944 
to  1000  fine. 

The  gold  is  next  melted  into  bars,  and  the  argentic 
chlorid  is  reduced  to  metallic  silver  by  placing  it 
between  two  wrought- iron  plates,  and  then  immersing 
the  whole  in  a  vessel  of  water  acidulated  with  sulfuric 
acid,  when,  after  a  few  hours,  the  silver  will  all  be 
reduced.  It  generally,  however,  contains  a  small 
percentage  of  gold,  which  may  be  recovered  either  by 
again  dissolving  the  silver  with  nitric  acid,  when  the 
gold  will  be  found  at  the  bottom  of  the  vessel ;  or  it 
may  be  separated  by  fusing  the  argentic  chlorid,  to 
which  is  added  a  small  quantity  of  potassic  carbonate 
for  the  purpose  of  reducing  a  little  metallic  silver. 
The  latter,  in  subsiding  through  the  argentic  chlorid, 
reduces  the  gold,  which  was  probably  combined  with 
the  chlorin.  While  yet  hot  and  in  a  fluid  state,  the 
argentic  chlorid  is  poured  off  and  reduced  as  before, 
when  it  will  be  found  free  of  gold.  The  little  button 
which  subsides  to  the  bottom  of  the  vessel  will  be 
found  to  consist  of  gold,  the  reduced  silver,  and  some 
adherent  argentic  chlorid.  The  latter  must  be  again 
decomposed  by  fusing  with  potassic  carbonate.  Theo- 
retically, one  cubic  foot  of  chlorin  will  convert  eight 
and  a  quarter  ounces  of  silver  into  argentic  chlorid, 


GOLD.  129 

but  in  practice  about  twice  that  quantity  is  required. 
Thus  far  the  use  of  chlorin  for  refining  upon  a  large 
scale  has  proved  to  be  much  more  economical  and 
expeditious  than  -the  humid  process,  the  time  required 
to  part  three  hundred  ounces  in  one  furnace  being 
about  two  hours,  and  the  average  cost  four  cents  per 
ounce. 

Treatment  of  Brittle  Gold. — The  slightest  admix- 
ture of  such  metals  as  arsenic,  antimony,  tin,  lead,  etc., 
is  sufficient  to  seriously  impair  the  ductility  of  gold. 
In  1856  the  coining  operations  of  the  mint  of  England 
were  much  embarrassed  by  the  importation  of  brittle 
gold,  in  which  the  contaminating  metal  did  not  exceed 
the  Y920  Part  °f  the  mass.  To  purify  the  gold  it  was 
exposed  while  in  a  state  of  fusion  to  a  stream  of 
chlorin  gas,  which  removed  the  deleterious  substances 
by  converting  them  into  volatile  chlorids.  The  tough- 
ness of  gold  may  also  be  restored  by  throwing  a  small 
quantity  of  corrosive  sublimate  on  the  surface  of  the 
molten  metal,  the  vapor  of  which  converts  the  metallic 
impurities  into  chlorids,  which  are  volatilized.  In  the 
dental  laboratory  gold  is  liable  to  become  contaminated 
with  small  particles  of  lead  or  zinc.  These  may  be 
effectually  removed  by  melting  with  a  mixture  of 
potassium  nitrate  and  borax,  when  the  foreign  metals 
will  be  oxidized  and  dissolved  in  the  slag.  Another 
process  consists  in  adding  to  the  melted  mass  about 
10  per  cent,  of  black  oxid  of  copper.  But  this  plan 
is  objectionable,  because  the  crucible  is  liable  to  become 
much  corroded,  and  even  perforated,  and  the  standard 
fineness  of  the  gold  lowered,  by  a  portion  of  the 
copper  being  reduced  to  the  metallic  state. 

The  gold  of  this  country  is  often  found  to  contain 


I30  DENTAL    METALLURGY. 

iridium,  the  presence  of  which  greatly  impairs  the 
metal  for  coinage  and  other  purposes.  The  little  hard 
grains  occasionally  met  with  in  gold,  upon  which  the 
file  makes  no  impression,  consist  of  iridium,  or  a 
native  alloy  of  osmium  and  iridium,  and  are  not  com- 
bined with  the  gold,  but  merely  disseminated  through 
it.  The  only  dry  method  of  separating  iridium  from 
gold  consists  in  alloying  the  latter  with  three  times  its 
weight  of  silver,  by  which  means  the  specific  gravity 
of  the  metal  is  so  much  lowered  that  the  iridium, 
which  is  very  infusible  and  of  a  specific  gravity  of 
2 1. 1,  will  subside  to  the  bottom  of  the  crucible,  when 
the  gold  and  silver  alloy  may  be  poured  or  ladled  off. 
As  some  gold  will  remain  with  the  residue,  more  silver 
must  be  melted  with  it,  the  operation  being  repeated 
several  times,  until  all  the  gold  is  removed .  What  is 
left  is  then  acted  upon  by  sulfuric  acid  to  dissolve  the 
silver,  when  the  iridium  and  some  finely-divided  gold 
will  be  left.     These  may  be  separated  by  washing. 

Iridium  may  also  be  separated  from  gold  by  the 
wet  process.  The  gold  is  melted  with  three  times  its 
weight  of  silver,  and  granulated  to  insure  admixture. 
The  alloy  is  then  treated  with  nitric  acid,  which 
dissolves  the  silver,  leaving  the  gold  and  iridium  at 
the  bottom  of  the  vessel.  The  gold  may  now  be  acted 
upon  by  nitro-hydrochloric  acid.  The  iridium  may 
then  be  collected  and  washed  to  free  it  from  any  por- 
tion of  the  gold.  The  latter  may  be  recovered  from 
its  solution  by  precipitation,  oxalic  acid  or  sulfurous 
acid  being  usually  employed. 

Preparation  of  Chemically- Pure  Gold. — None  of  the 
methods  which  have  been  described  can  always  be 
relied  upon  to  afford  absolutely  pure  gold.     When 


GOLD.  131 

nitric  acid  is  employed  in  the  quartation  process,  gold 
may  be  obtained  from  993  to  997  parts  in  the  1000, 
while  sulfuric  acid  will  frequently  yield  gold  up  to  99S 
thousandths. 

Assays  made  by  Messrs.  DuBois  and  Eckfeldt, 
assayers  at  the  United  States  Mint,  of  some  of  the 
most  prominent  foils  give  the  following  results  :* 

No.  1.  Abbey's  Non-cohesive      .  998.8  998.7 

"     2.  Wolrab's   ....  999.2  999.3 
"     3.  Quarter  Century,  S.  S.  W. 

Dental  Mfg.  Co.     .         .  999  1  999.1 

"     4.  Rowan's  Decimal  foil       .  999.9  999-8 

There  are  several  methods  by  which  chemically-pure 
gold  may  be  obtained.  Usually,  ordinary  refined  gold, 
obtained  by  one  of  the  methods  above  described, 
is  dissolved  in  nitro-hydrochloric  acid.  The  excess 
of  acid  is  driven  ofT,  and  alcohol  and  chlorid  of  potas- 
sium are  added  for  the  purpose  of  precipitating  plati- 
num, if  any  is  present.  The  chlorid  of  gold  is  then 
dissolved  in  pure  distilled  water,  until  each  gallon 
does  not  contain  more  than  half  an  ounce  of  the 
chlorid.  Any  silver  present  will  be  converted  into 
argentic  chlorid,  which  will  settle  to  the  bottom  of  the 
vessel,  after  which  the  supernatant  liquid  should  be 
carefully  removed  by  means  of  a  syphon.  The  gold 
may  be  precipitated  by  a  stream  of  carefully-washed 
sulfurous  anhydrid,  or  by  the  addition  of  oxalic  acid. 
The  precipitated  metal  is  washed  with  dilute  hydro- 
chloric acid,  distilled  water,  ammonia  water,  and  again 
with  distilled  water,  and  is  then  ready  for  melting. 
This  is  done  in  a  clay  crucible,  with  a  small  portion  of 
bisulfate  of  potash  and  borax.    The  melted  metal  should 

*  American  System  of  Dentistry. 


I32  DENTAL    METALLURGY. 

be  poured  into  a  stone  ingot-mold.  By  this  method 
gold,  of  which  the  purity  was  999.96,  has  been  pre- 
pared, the  precipitant  being  oxalic  acid  ;  but  gold 
precipitated  by  that  agent  from  an  acid  solution  con- 
taining copper  is  always  contaminated  with  cupric 
oxalate,  to  avoid  which  the  solution  should  be  heated, 
with  the  addition  of  potash,  when  a  soluble  double 
oxalate  of  copper  and  potash  is  formed,  leaving  the 
gold  in  the  pure  state. 

The  aqua  regia  used  in  the  preparation  of  chemically- 
pure  gold  should  consist  of  two  parts  of  hydrochloric 
and  one  part  of  nitric  acid.  The  specific  gravity  of 
the  former  should  be  about  1.16,  and  of  the  latter  1.45. 
Each  ounce  of  gold  will  require  for  its  solution  about 
three  and  one-half  ounces  of  the  mixed  acids.  The 
action  of  this  upon  the  metal  will  in  the  beginning  be 
quite  energetic,  but  as  the  solution  approaches  satura- 
tion the  application  of  moderate  heat  is  required  to 
dissolve  the  last  portion  of  the  gold.  The  greatest 
care  must  be  exercised  in  the  separation  of  the  gold 
solution  from  the  argentic  chlorid,  which  subsides  to 
the  bottom  of  the  vessel,  and  also  to  rid  the  liquid  of 
the  small  portion  of  silver  held  in  solution  by  the  acid. 
The  solution  is  cautiously  transferred  to  an  evaporating 
dish  by  means  of  a  syphon,  and  heat  is  applied,  and 
as  the  bulk  is  gradually  reduced  by  evaporation  more 
argentic  chlorid  will  be  separated  and  deposited  at  the 
bottom.  The  supernatant  liquid  should  again  be 
carefully  poured  or  syphoned  off,  and  this  should  be 
repeated  as  often  as  the  residue  appears  in  the  dish. 
When  the  solution  has  become  viscid  and  of  a  deep- 
ruby  color,  the  heat  is  discontinued,  and  the  auric 
chlorid  soon  crystallizes  in  a  mass  of  prismatic  forms. 


GOLD.  133 

It  should  then  be  dissolved  and  largely  diluted  with 
distilled  water,  acidulated  by  a  few  drops  of  hydro- 
chloric acid,  and,  after  standing  for  a  few  days  to 
permit  a  further  subsidence  of  argentic  chlorid,  it 
should  be  filtered,  when  it  is  ready  for  precipitation. 
This  may  be  accomplished  by  quite  a  number  of 
different  reagents,  but  the  form  of  the  precipitated 
metal  depends  upon  the  nature  of  the  precipitant,  and 
it  may  be  thrown  down  in  a  spongy  condition,  in  sheets 
resembling  foil,  as  a  powder,  in  a  more  or  less  crys- 
talline state,  and  in  scales.  The  affinity  of  gold  for 
other  bodies  is  so  weak  that  care  must  be  observed 
lest  partial  reduction  be  effected  by  merely  adventi- 
tious conditions.  The  highly  diluted  neutral  solution 
of  the  trichlorid  just  described  is  quite  liable  to  such 
accidents  ;  indeed,  it  may  occur  from  exposure  to  air, 
atmospheric  nitrogen  probably  being  the  active  agent. 
The  addition  of  pure  water  to  such  a  solution  may 
also  cause  slight  precipitation,  but  the  dilute  solution 
may  be  protected  from  premature  precipitation  by 
acidulation  with  a  small  quantity  of  hydrochloric  acid. 
The  best  agents  for  the  precipitation  of  gold  are 
oxalic  acid,  sulfurous  acid,  and  ferrous  sulfate.  Oxalic 
acid  will  precipitate  several  forms  of  gold,  from 
sponge-like  masses  to  the  different  crystalline  or 
powdery  forms.  Its  action  is,  however,  slower  than 
the  others,  and  it  requires  to  be  slightly  heated.  The 
reaction  is  shown  in  the  following  equation  : 
2AuCl:;+3H2C,01=6HC14-6CO,-|-2Au. 

The  chlorin  of  the  auric  chlorid  unites  with  the 
hydrogen  of  the  oxalic  acid  to  form  hydrochloric  acid, 
the  copious  evolution  of  gas  noticed  during  the  pre- 
cipitation being  the  escape  of  the  carbonic  acid  formed 


134  DENTAL    METALLURGY. 

by  the  remaining  elements  of  the  oxalic  acid.     The 
gold  is  thus  set  free. 

The  so-called  "shredded  gold,"  somewhat  exten- 
sively used  by  dentists  in  filling  teeth  during  1867-68, 
was  produced  by  the  addition  of  sugar  or  gum  arabic 
to  an  acid  solution  of  gold.  The  exact  modus  operandi 
is  as  follows  :  The  pure  gold  is  dissolved  in  nitro- 
hydrochloric  acid,  and,  without  evaporating  the  solu- 
tion, it  is  diluted  by  the  addition  of  about  two-thirds 
its  bulk  of  pure  water.  Clean  gum  arabic,  dissolved 
in  boiling  water  to  the  amount  of  one-third  the  bulk 
of  the  gold  solution,  is  added  to  the  latter,  and  the 
whole  poured  into  a  glass  matrass  or  evaporating- dish 
and  placed  over  a  steam  bath.  When  the  proper 
temperature  is  attained,  gold  in  the  form  of  leaves, 
shreds,  or  fibers  will  be  observed  floating  in  the  liquid. 
When  these  become  sufficiently  coherent  to  admit  of 
removal  they  are  lifted  out  by  a  vulcanized  rubber 
spoon  attached  to  a  glass  rod,  and  placed  in  a  filter. 
This  operation  is  continued  until  the  gum  arabic  or 
sugar,  assisted  by  heat,  has  caused  the  precipitation  of 
all  the  gold  held  in  solution.  The  web-like  masses 
are  then  thoroughly  washed,  dried,  and  heated  to  dull 
redness.  As  the  elements  entering  into  the  composi- 
tion of  sugar  and  gum  arabic  are  identical  with  those 
of  oxalic  acid,  the  reaction  is  probably  the  same  as 
that  which  occurs  when  the  latter  is  employed  as  the  pre- 
cipitant. With  care  in  the  application  of  the  proper 
amount  of  heat,  the  action  of  precipitants  of  this  class 
is  capable  of  regulation,  thus  affording  uniform  results. 

When  the  precipitated  gold  is  intended  for  plate  or 
bars,  it  should  be  well  washed,  and  fused  in  a  perfectly 
new  crucible. 


GOLD.  135 

Sulfurous  acid  precipitates  gold  generally  in  the 
form  of  a  scaly  metallic  powder  ;  hence  it  does  not 
afford  masses  sufficiently  coherent  or  sponge-like  for 
use  as  a  filling-material  for  the  dentist.  The  reaction 
which  takes  place  is  thus  explained  : 

2AuC]3+3H20  +  3H.,S03=6HCl+3H2S04+2Au. 

The  water  present  is  decomposed,  its  hydrogen 
uniting  with  the  chlorin  of  the  auric  chlorid  to  form 
hydrochloric  acid.  The  oxygen  of  the  water,  attracted 
to  the  sulfurous  acid,  converts  it  into  sulfuric  acid,  and 
the  gold  is  thus  liberated. 

Ferrous  sulfate  precipitates  gold  in  the  form  of  a 
light-brown  powder.  Of  the  sulfate  crystals  about 
four  times  the  weight  of  the  gold  is  dissolved  in  water. 
This  is  added  to  the  auric  solution.  After  the  finely- 
divided  gold  has  entirely  subsided,  it  should  be  boiled 
several  times  in  dilute  hydrochloric  acid,  in  order  to 
free  it  from  all  traces  of  the  iron  with  which  it  is  liable 
to  be  contaminated.  The  interchange,  which  in  this 
reaction  results  in  the  liberation  of  the  gold,  is  ex- 
pressed by  the  following  equation  : 

2AuCl3-f6FeS04=Fe2Cl64-2(Fe23S04)4-2Au. 

The  ferrous  salt  parts  with  a  portion  of  the  iron  to 
the  chlorin  of  the  gold  salt,  thus  forming  ferric  chlorid 
and  ferric  sulfate,  while  the  gold  is  liberated. 

As  stated  before,  the  reduction  of  the  metal  from 
the  trichlorid  may  be  effected  (in,  however,  a  less 
satisfactory  manner)  by  many  different  reagents,  some 
of  which  are  purely  elementary.  Thus,  sulfur,  sele- 
nium, carbon  (charcoal),  and  phosphorus,  each,  when 
introduced  into  a  heated  solution,  becomes  coated 
with  a  film  of  metallic  gold.     Reduction  may  also  be 


136  DENTAL     METALLURGY. 

accomplished  by  some  of  the  gaseous  bodies  contain- 
ing hydrogen.  Thus,  gold  may  be  precipitated  by 
arseniureted  and  antimoniureted  hydrogen.  Many  of 
the  base  metals,  such  as  bismuth,  zinc,  etc.,  also  reduce 
gold  from  solution  in  the  form  of  a  brown  powder. 
It  is  also  reduced  on  a  platinum  pole  by  the  electrical 
current.  In  this  way  the  beautiful  form  of  gold  made 
by  A.  J.  Watts,  of  New  York,  is  produced.  In  a 
solution  of  auric  chlorid  plates  of  pure  gold  are  sus- 
pended. These  are  connected  with  a  battery,  so  that 
as  the  solution  loses  its  gold  by  deposition  of  the 
metal  it  is  re-supplied  from  the  suspended  plates.  By 
this  means  large  masses  of  perfectly  pure  crystal  gold 
may  be  obtained. 

Gold  may  also  be  precipitated  by  some  of  the 
metallic  salts,  of  which  nitrate  of  mercury  and  chlorid 
of  antimony  may  be  named  as  examples.  Quite  a 
number  of  organic  substances  will  also  precipitate  it, 
a  prominent  example  of  which  is  gallic  acid.  The 
tartrate,  citrate,  and  acetate  of  potassium  will  also 
reduce  it.  Some  of  the  members  of  this  class,  how- 
ever, require  the  addition  of  heat,  and  to  obtain 
prompt  action  with  these  agents  the  solution  should  be 
quite  neutral. 

Alloys  of  Gold. — The  most  important  alloys  are 
those  with  silver  and  copper.  The  coinage  of  the 
present  day,  from  which  the  dentist  usually  obtains 
his  plate,  is  mainly  an  alloy  of  gold  and  copper,  in  the 
proportion  of  900  parts  of  gold  in  1000.  Gold 
coins  were  first  introduced  in  England  by  Henry  III, 
in  1257.  They  were  of  pure  gold.  Edward  III,  in 
1345,  established  a  standard  of  994.8,  and  in  1526 
Henry  VIII  issued  crowns  of  the  double-rose,  of  the 


GOLD.  137 

standard  916.6.  In  1544  the  standard  of  all  gold 
coins  was  reduced  to  916.6,  and  again  in  1548  to 
833.4.  Mary  restored  the  old  standard,  994.8.  In 
Elizabeth's  reign  coins  of  both  standards  (916.6  and 
994.8)  were  issued.  In  America  the  standard  of  gold 
coin  is  900. 

Alloys  of  gold  for  bases  for  artificial  dentures  should 
be  of  such  fineness  as  will  enable  them  to  resist  chemi- 
cal action  of  the  fluids  of  the  mouth,  while  at  the 
same  time  they  should  possess  the  requisite  hardness, 
strength,  and  elasticity.  These  properties  are  usually 
conferred  by  the  addition  of  copper  and  silver,  or 
either  of  these  metals  singly  ;  or  by  copper,  silver, 
and  platinum.  The  quality  of  gold  which  is  to  be 
introduced  into  the~mouth  should,  as  a  rule,  be  of  a 
standard  of  fineness  not  less  than  eighteen  carats. 
Care,  however,  should  be  observed  in  remelting  the 
scraps  and  filings  of  the  drawer,  that  the  grade  of  the 
gold  be  not  lowered  by  the  admixture  of  old  plates, 
backings,  etc. ,  containing  portions  of  solder.  Indeed, 
much  the  safer  rule  is  to  remelt  only  new  scraps,  on 
which  no  solder  has  been  used.  The  scraps  and 
filings  of  doubtful  quality  may  be  sent  to  the  mint  for 
coinage,  the  charge  for  which,  on  the  one  hundred 
dollars  (the  minimum  amount  received  by  the  United 
States  Mint),  is  less  than  1  per  cent. 

Gold  exceeding  nineteen  carats  in  fineness  will 
generally  be  found  too  soft  and  yielding  for  use  in  the 
mouth.  The  amount  of  force  which  the  plate  must 
sustain  in  mastication  is  much  greater  than  might  be 
supposed  ;  hence,  if  the  degree  of  purity  of  the  allov 
be  too  high,  the  requisite  amount  of  rigidity  and 
strength  will  be  wanting,  and  the  plate  will  soon  bend 

10 


138 


DENTAL  METALLURGY. 


to  such  an  extent  that  it  will  no  longer  fit  the  mouth. 
This  difficulty  may,  however,  be  avoided  in  the  higher 
grades  of  gold  plate  intended  for  dental  purposes  by 
a  slight  admixture  of  platinum,  by  which  much  greater 
tenacity  is  obtained  ;  otherwise  the  plate  will  require 
strengthening  by  doubling  at  such  points  as  are  most 
liable  to  bend.  Plates  for  partial  cases  necessarily 
require  a  great  deal  of  strengthening.  This  adds 
considerably  to  the  expenditure  of  time  and  labor  in 
the  construction  of  the  plate.  It  increases  its  weight, 
and  does  not  always  render  the  plate  sufficiently  rigid 
to  withstand  the  force  of  mastication. 

Gold  plate  suitable  for  dental  purposes  may  be 
prepared  according  to  the  following  formulae,  from 
Richardson's  "  Mechanical  Dentistry  :" 

Gold  Plate  18  Carats  Fine. 


No. 

i. 

No. 

2. 

Pure  Gold    . 

. 

18  dwts. 

Gold  Coin    . 

. 

20  dwts. 

Pure  Copper 

. 

4     " 

Pure  Copper 

2      " 

Pure  Silver  . 

• 

2      " 

Gold  Plate  19 

Pure  Silver  . 

Carats  Fine. 

• 

2      " 

No. 

3- 

No. 

4- 

Pure  Gold     . 

. 

19  dwts. 

Gold  Coin    . 

, 

20  dwts. 

Pure  Copper 

. 

3      " 

Pure  Copper 

. 

25  grs. 

Pure  Silver  . 

• 

2      " 
Gold  Plate  20 

Pure  Silver  . 

Carats  Fine. 

• 

40+ grs. 

No. 

5- 

No. 

6. 

Pure  Gold     . 

, 

20  dwts. 

Gold  Coin    . 

, 

20  dwts. 

Pure  Copper 

. 

2       " 

Pure  Copper 

18  grs. 

Pure  Silver  . 

• 

2        " 

Gold  Plate  21 

Pure  Silver  . 

Carats  Fine. 

• 

20+  grs. 

No. 

7- 

No 

8. 

Pure  Gold 

21  dwts. 

Gold  Coin    . 

# 

20  dwts. 

Pure  Copper 

2        " 

Pure  Silver  . 

. 

13  4-  grs. 

Pure  Silver  . 

i    dwt. 

GOLD.  139 


No 

■  9- 

Gold  Coin 

. 

20  dwts. 

Pure  Copper    . 

. 

6grs. 

Pure  Platinum 

• 

7t  grs. 

Gold  Plate  22 

Carats  Fi 

ne. 

No. 

IO. 

Pure  Gold 

. 

22  d\VtS. 

Fine  Copper    . 

. 

i  dwt. 

Pure  Silver 

. 

18  grs. 

Pure  Platinum 

^          # 

6     " 

Gold  Plate  18  Carats  Fine. 
No.  II. 

United  States  Gold  Coin  (#60)  .        .     64^  dwts. 
Pure  Silver 13        " 

On  account  of  its  greater  strength  and  power  of 
resisting  chemical  action  of  the  fluids  of  the  mouth, 
many  dentists  prefer  to  use  gold  plate  twenty  or 
twenty- one  carats  fine,  in  which  the  reducing  con- 
stituents are  copper  and  platinum,  the  following 
formula  being  an  example  : 

Gold  Coin        .  .20  dwts. 

Pure  Platinum  .         .     10  grs. 

The  union  of  platinum  with  gold  yields  an  alloy 
possessing  great  strength  and  considerable  elasticity. 
Such  an  admixture,  however,  has  its  disadvantages. 
Owing  to  its  increased  strength  and  stiffness,  a  much 
thinner  and  lighter  plate  may  be  employed  without  the 
additional  labor  and  cost  of  doubling  the  plate  at 
what,  in  partial  cases  composed  of  ordinary  18-,  19-, 
or  20-  carat  gold,  would  be  weak  points.  It  may  also 
be  justly  claimed  for  gold  alloyed  with  platinum  that 
it  will  perfectly  resist  the  action  of  the  fluids  of  the 
mouth.     On  the  other  hand,  the  richness  of  color  of  the 


I40  DENTAL    METALLURGY. 

gold  is  always  more  or  less  impaired  by  the  admixture 
of  platinum.  But  perhaps  the  greatest  objection  to 
be  urged  against  the  employment  of  platinum-gold  is 
the  increased  difficulty  of  swaging  a  plate  composed 
of  it  so  that  it  shall  perfectly  conform  to  all  the  de- 
pressions and  irregularities  of  the  model.  Having 
invariably  found,  when  the  alloy  contained  any  con- 
siderable percentage  of  platinum,  that  the  ordinary 
method  of  swaging  between  zinc  and  lead  was  not 
effective,  the  author  has  for  more  than  twenty  years 
employed  zinc  for  counter-dies  as  well  as  for  dies  ;  this 
entirely  overcomes  any  difficulty  in  swaging.* 

Gold  for  use  in  the  formation  of  clasps  should  always 
contain  sufficient  platinum  to  render  it  much  more 
elastic  than  the  alloys  usually  employed  in  the  plate 
or  base,  so  that  on  the  application  of  force  upon  the 
denture  in  the  act  of  mastication  the  clasp,  though  it 
may  yield  slightly,  will  always  spring  together  again 
and  accurately  embrace  the  tooth  which  it  surrounds. 
In  the  perfect  adjustment  of  clasps  to  remaining  teeth 
the  following  points  are  of  importance  :  First,  the 
model  must  be  as  accurate  as  a  plaster  impression  will 
afford  ;  second,  the  clasp  should  be  thrown  around 
the  thickest  or  most  prominent  part  of  the  tooth  ; 
third,  the  clasp  should  be  so  arranged  as  to  fit  ac- 
curately the  convexity  of  the  tooth.  To  successfully 
accomplish  this  the  gold,  of  about  No.  26  of  the 
standard  gauge,  should  be  cut  by  pattern,  and  before 
any  attempt  is  made  to  fit  it  to  the  tooth  it  should  be 
bent  with  the  clasp-benders  to  correspond  with  the 
rounded  surfaces  of  the  natural  tooth.  Lastly,  the 
contact  of  the  clasps  with  the  tooth  should  be  uniform. 

*  See  chapter  on  "  Zinc." 


GOLD.  141 

The  ends  of  the  clasp  should  be  free,  and  it  should  be 
attached  to  the  plate  at  one  point,  so  that  but  little  of 
its  circumference  shall  be  included  in  the  union  ;  other- 
wise, if  a  large  proportion  of  the  clasp  be  soldered 
fast  to  the  plate,  much  of  the  quality  of  elasticity  will 
be  lost. 

The  following  formula?  will  afford  alloys  of  twenty 
carats'  fineness,  suitable  for  clasps,  backings,  etc.,  wher- 
ever elasticity  and  additional  strength  are  required  : 

Formula  No.  1.  Formula  No.  2. 

Pure  Gold     .  .  20  dwts.  j  Coin  Gold     .  .     20  dwts. 

Pure  Copper  .  2      "          Pure  Copper  .       8  grs. 

Pure  Silver  .  .  1  dwt.        Pure  Silver  .  .     10     " 

Pure  Platinum  .  1   . "         Pure  Platinum  .20     " 

Alloys  of  Gold  employed  in  Dentistry  as  Solders.  — 
These  are  a  class  of  alloys  formed  of  the  metal  to  be 
united,  the  fusing-point  of  which  is  reduced  by  the 
addition  of  silver,  copper,  and  brass.* 

No.  1.    14  Carats  Fine. 
American  Gold  Coin,  5 10.00 
Pure  Silver     .        .     4  dwts. 
Pure  Copper  .        .2      " 

No.  3.     14  Carats  Fine. 
Pure  Silver  .         .     i\  dwts. 


No.  2.    14  Carats  Fine. 
American  Gold  Coin,  16  dwts 
Pure  Copper,  3  dwts.  18  grs. 
Pure  Silver     .        .     5  dwts. 


No.  4.     15 

Carats  Fine. 

Gold  Coin     . 

6  dwts 

Pure  Silver  . 

•     30  grs. 

Pure  Copper 

.     20    " 

Brass     . 

.     10    " 

Pure  Copper         .     20  grs. 
Pure  Zinc      .         .    35    *' 
iS-carat  Gold  Plate 

(formula  No.  11)    20  dwts.   | 

No.  5.    16  Carats  Fine.  No.  6.    16  Carats  Fine. 

Pure  Gold  .    11  dwts.  Pure  Gold  .  11  dwts.  12  grs. 

Pure  Silver       3      "      6  grs.      Pure  Silver      3     " 
Pure  Copper    2      "      6    "        Pure  Copper   1  dwt.  12    " 

Pure  Zinc  .         .         .  12    " 

*  See  page  35. 


142 


DENTAL  METALLURGY. 


No.  7.    18  Carats  Fine. 

Gold  Coin    .  .  30  parts. 

Pure  Silver  .  .  4     " 

Pure  Copper  .  1  part. 

Brass    .        .  .  1     " 


No.  8.    20  Carats  Fine,  for  Crown 
and  Bridge-work. 

American  Gold  Coin  (21.6 
carats  fine)  $10  piece,  258  grs. 
Spelter  Solder      .     20.64  " 


No.  9.    20  Carats  Fine,  same  use  as  No.  8. 

Pure  Gold        .  .      5  dwts. 

Pure  Copper    .  .      6  grs. 

Pure  Silver      .  .  12    " 

Spelter  Solder  .      6    " 

No.  10.    20  Carats  Fine,  for  Crown-  and  Bridge-work. 

Zinc        .        .        .     i\  grs. 
Pure  Gold      .        .     20    " 
Silver  Solder         .      3    " 

No.  11.    Dr.  C.  M.  Richmond's  Solder  for  Bridge-work. 
Gold  Coin        .         .     5  dwts. 
Fine  Brass  Wire      .     1  dwt. 

No.  12.    Dr.  Low:s  Formula  for  Solder  in  Crown-  and  Bridge- work. 
19  Carats  Fine. 

Coin  Gold        .        .     1  dwt. 
Copper     .        .        .2  grs. 
Silver       .        .        .     4    " 

Dr.  W.  H.  Dorrance  prepares  an  alloy  of 

Pure  Silver      .        .     1  dwt. 
Pure  Zinc         .        .     2  dwts. 
Pure  Copper   .         .     3     " 

With  this  alloy  he  forms  the  different  grades  of  gold 
solders.  To  form  a  solder  suitable  for  bridge-work, 
of  20  carats  fine,  he  melts  4  grains  of  the  alloy  with  20 
grains  of  pure  gold. 

Spelter  solder,  composed  of  equal  parts  copper  and 
zinc,  is  sometimes  employed  as  a  constituent  in  the 
preparation  of  gold  solders  for  the  purpose  of  reducing 


GOLD.  143 

the  fusing-point.     Thus,  some  dentists  use  an  alloy 
composed  of 

iS-Carat  Gold 6  dwts. 

Granulated  Spelter  Solder         .        .      6  grs. 

An  alloy  of  this  composition  is  exceedingly  brittle, 
and  hence  difficult  to  roll  into  plate  without  breaking 
into  many  pieces.  Its  color  is  good,  but  the  author 
has  noticed  that  the  surface  of  such  solders,  after  flow- 
ing, is  apt  to  be  pitted  with  small  holes,  and  has  not 
the  solid  and  uniform  appearance  that  is  desirable. 
This  may  be  due  to  the  oxidation  and  escape  of  some 
of  the  zinc. 

Methods  of  Reducing  Gold  to  a  Lower  or  Higher 
Standard  of  Fineness,  and  of  Determining  the  Carat 
of  any  given  Alloy. — The  gold  alloys  used  in  the 
laboratory  are  generally  made  from  pure  gold  or  gold 
coin,  the  standards  of  which  are  definitely  fixed.  A 
few  simple  rules  are  here  given,*  by  which  the  opera- 
tor may  readily  determine  the  quantity  of  alloy  neces- 
sary to  reduce  either  coin  or  pure  gold  to  any  desired 
standard. 

To  ascertain  the  carat  of  any  given  alloy,  multiply 
24  by  the  weight  of  gold  in  the  alloyed  mass,  and 
divide  the  product  by  the  weight  of  the  mass.  The 
quotient  is  the  carat  sought.  For  example,  take  the 
following  ; 

Pure  Gold 18  parts. 

Copper        4 

Silver 2 


24 


*  Richardson's  Mechanical  Dentistry. 


144  DENTAL     METALLURGY. 

The  result  may  be  thus  expressed  : 

24  X  18  -=-  24  =  18  carats. 

To  reduce  gold  to  a  required  carat,  multiply  the 
weight  of  gold  used  by  24,  and  divide  the  product  by 
the  required  carat.  The  quotient  is  the  weight  of  the 
mass  when  reduced,  from  which  subtract  the  weight 
of  the  gold  used,  and  the  remainder  is  the  weight  of 
the  alloy  to  be  added. 

To  raise  gold  from  a  lower  to  a  higher  carat,  multi- 
ply the  weight  of  the  alloyed  gold  used  by  the  num- 
ber representing  the  proportion  of  alloy  in  the  given 
carat ;  divide  the  product  by  the  figures  representing 
the  quantity  of  alloy  in  the  required  carat.  The 
quotient  is  the  weight  of  the  mass  when  reduced  to 
the  required  carat  by  adding  fine  gold.  Thus,  to 
raise  one  pennyweight  of  16-carat  gold  to  18  carats, 
the  numbers  representing  the  proportions  of  alloys  are 
obtained   by   subtracting    18    and  16  from  24.     The 

statement  is — 

6:8::i:ii; 

from  which  it  will  be  seen  that,  to  raise  one  penny- 
weight of  16-carat  gold  to  18  carats,  one-third  of  a 
pennyweight  of  pure  gold  must  be  added  to  it. 

Again,  if  instead  of  using  pure  gold  we  desire  to 
raise  the  fineness  of  one  pennyweight  of  16-carat 
gold  to  that  of  18,  by  the  addition  of,  say  22-carat 
gold,  the  numbers  representing  the  proportions  of  the 
alloy  would  be  found  by  subtracting,  in  the  example 
given,  16  and  18  from  22,  the  result  being — 

4  :6  :  :  1  :  ii. 

Hence  each  pennyweight  of  16-carat  gold  would  re- 


GOLD. 


145 


quire  a  half-pennyweight  of  22-carat  gold  to  raise  it 
to  18  carats. 

The  fineness  of  gold  may  be  expressed  in  decimals 
or  in  parts  called  carats.  The  former  is  the  system 
employed  at  the  United  States  Mint  and  by  metal- 
lurgists and  chemists,  while  the  latter  is  the  usual 
method  of  expressing  the  grade  of  alloys  of  gold 
among  dentists  and  jewelers.  The  following  table 
will  show  the  relation  of  one  to  the  other : 


Carats. 

Decimals. 

Pure  gold 

24 

IOOO 

English  coin 

22 

916.6 

American  coin  . 

21.6 

900 

Dentists'  gold    . 

20 

833-3 

«<            <  < 

I9.2 

800 

Jewelers'  gold,  best  . 

18 

750 

"    good 

15 

625 

"    low  grade 

12 

500 

Common  jewelers'  solder 

8 

333-3 

There  are  many  different  alloys  used  in  the  arts. 
The  greenish  alloy  used  by  jewelers  contains  70  per 
cent,  of  silver  and  30  percent,  of  gold.  "  Blue  gold" 
is  stated  to  contain  75  per  cent,  of  iron.  The  Jap- 
anese employ  a  compound  of  gold  and  silver,  the 
standard  of  which  varies  from  350  to  500.  This  alloy 
is  exposed  to  the  action  of  a  mixture  of  plum-juice, 
vinegar,  and  sulfate  of  copper.  They  also  possess  a 
number  of  bronzes,  in  which  tin  and  zinc  are  replaced 
by  gold  and  silver.  The  alloy  known  as  shiya-ku-do, 
extensively  used  for  sword  ornaments,  contains  70 
per  cent,  of  copper  and  30  per  cent,  of  gold. 


I46  DENTAL     METALLURGY. 

Alloys  of  Gold  and  Silver. — The  density  of  those 
natural  alloys,  the  composition  of  which  varies  from 
AuAg6  to  Au6Ag,  is  greater  than  that  calculated  from 
the  densities  of  the  constituent  metals.  Gold  and 
silver  unite  in  all  proportions,  affording  alloys  of  several 
tints,  ranging  from  the  color  of  silver  to  that  of  gold. 
By  the  addition  of  silver  the  hardness  of  gold  is  in- 
creased, and  it  is  rendered  more  fusible,  while  its  mal- 
leability is  not  materially  diminished.  Gold  as  found 
in  nature  always  contains  silver,  and  all  specimens  of 
native  silver  will  likewise  be  found  to  contain  gold. 

Gold  and  Platinum. — Equal  weights  of  the  two 
metals  yield  an  alloy  of  good  malleability,  with,  how- 
ever, some  dullness  of  color.  An  excess  of  platinum 
renders  the  alloy  infusible  in  an  ordinary  blast-furnace. 
One  part  of  platinum  to  9.5  of  gold  will  afford  an 
alloy  of  the  same  density  as  platinum. 

Gold  and  Tin. — Alloys  of  tin  and  gold  are  hard  and 
brittle,  and  the  combination  is  attended  with  contrac- 
tion. Thus,  the  alloy  SnAu  has  a  density  14.243, 
instead  of  14.828,  as  indicated  by  calculation. 

Gold  and  Mercury. — These  combine  at  all  tempera- 
tures, but  the  union  may  be  greatly  facilitated  by 
heating,  and  a  state  of  fine  division  still  further  assists 
the  process.  It  is  stated  that  an  amalgam  composed 
of  six  parts  of  mercury  to  one  of  gold  crystallizes  in 
four-sided  prisms,  and  that,  if  the  mercury  is  then  dis- 
tilled off,  the  gold  is  left  in  an  arborescent  state.  The 
operation  of  coating  the  surface  of  brass  or  copper 
objects  with  gold,  extensively  practiced  some  years 
ago,  and  known  as  "  fire-gilding,"  was  based  upon 
the  amalgamation  of  gold  and  mercury.  The  process 
is  as  follows  :    The  article  to  be  gilded  is  given  a  uni- 


GOLD.  147 

form  coating  of  an  amalgam  made  by  heating  six  parts 
of  mercury  with  one  part  of  gold,  with  the  surplus 
mercury  removed  by  squeezing.  The  operation  is 
greatly  facilitated  by  first  rubbing  the  surface  with  mer- 
curous  nitrate.  It  is  thus  given  a  superficial  layer  of 
mercury.  It  is  now  gently  heated  over  some  burning 
charcoal,  the  amalgam  in  the  mean  time  being  kept 
uniformly  distributed  over  the  surface  by  means  of  a 
soft  brush.  As  the  heat  is  continued  and  the  mercury 
is  gradually  driven  off,  the  surface  assumes  a  dull  yel- 
low color.  It  is  then  ready  for  polishing,  which  is 
accomplished  by  means  of  a  wheel-brush  moistened 
with  vinegar.  Verdigris  mixed  with  beeswax  is 
applied,  for  the  purpose  of  removing  any  remaining 
mercury  by  means  of  the  affinity  which  the  latter  has 
for  the  acetate  of  copper.  This  operation,  sometimes 
called  "water-gilding,"  is  so  dangerous  to  health,  in 
consequence  of  the  liability  of  the  operator  to  inhale 
the  volatilized  mercury,  that  it  has  been  almost  en- 
tirely superseded  by  electro-gilding. 

Gold  and  Copper  yield  a  class  of  alloys  of  a  reddish 
color  (between  gold  and  copper),  which  are  much 
harder  than  either  of  their  constituents.  The  mal- 
leability of  the  gold  is  not,  however,  much  affected 
by  admixture  of  copper,  provided  the  latter  is  pure. 
It  is  stated  that  seven  parts  of  gold  with  one  of  cop- 
per exhibits  the  greatest  degree  of  hardness  which  it 
is  possible  to  obtain  by  union  of  these  two  metals. 
In  the  American  coinage  the  alloy  is  chiefly  copper  ; 
hence  the  coins  are  red  in  color  and  very  hard. 

Gold  and  Palladium. — These  metals  are  stated  to 
alloy  in  all  proportions.  Chenevix  states  that  an  alloy 
composed  of  equal  parts  of  the  two  metals  is  gray  in 


I48  DENTAL     METALLURGY. 

color,  less  ductile  than  its  constituents,  and  has  the 
specific  gravity  of  11.08.  W.  Chandler  Roberts, 
assayer  of  the  Royal  Mint,  London,  states  that  an 
alloy  of  four  parts  of  gold  and  one  of  palladium  is 
white,  hard,  and  ductile.  According  to  Makins,  the 
merest  trace  of  palladium  with  gold  will  render  the 
latter  very  brittle.  Graham  has  shown  that  a  wire  of 
palladium  alloyed  with  from  twenty-four  to  twenty-five 
parts  of  gold  does  not  exhibit  the  remarkable  retrac- 
tion which,  in  pure  palladium,  attends  its  loss  of 
occluded  hydrogen. 

Gold  and  Zinc. — It  appears  that  these  two  metals 
possess  a  strong  affinity  for  each  other,  but  all  the 
alloys  of  zinc  and  gold  are  more  or  less  brittle,  accord- 
ing to  the  quantity  of  zinc  present.  Care  should, 
therefore,  be  observed  in  the  dental  laboratory,  where 
so  much  zinc  is  employed,  that  small  particles  of  it  do 
not  find  their  way  into  the  gold  filings. 

There  are  certain  other  metals  which,  when  mixed 
with  gold  in  quantities  as  small  as  the  1912Q  part  of  the 
mass,  render  it  quite  brittle  and  unworkable.  These 
are  bismuth,  lead,  antimony,  and  arsenic. 

Compounds  of  Gold. — Two  compounds  of  gold  with 
oxygen  have  been  obtained, — Au20  and  Au203, — but 
neither  of  them  is  of  any  great  practical  importance. 
The  chlorids  of  gold  correspond  in  composition  to  the 
oxids. 

Auric  chlorid,  or  trichlorid,  as  it  is  more  commonly 
called,  is  prepared  by  dissolving  gold  in  nitro-hydro- 
chloric  acid.  The  excess  of  acid  is  driven  off  by 
evaporation  at  a  temperature  not  greater  than  280°  F. 
(=138°  C.)  ;  otherwise  a  part  of  it  at  least  will  be 
converted  into  aurous  chlorid.     The  crystals  obtained 


GOLD.  149 

by  this  process  are  ruby-red  in  color,  and  very  de- 
liquescent. The  composition  of  auric  chlorid  is 
AuCl3 ;  atomic  weight,  303.1. 

Aurous  chlorid  is  obtained  by  heating  the  crys- 
tallized auric  chlorid  in  a  porcelain  evaporating-dish 
to  about  3470  F.  (=175°  C).  If  the  temperature  is 
carried  much  beyond  this  point,  say  to  3920  F. 
(=200°  C),  the  compound  will  be  decomposed  into 
metallic  gold  and  chlorin  gas.  Aurous  chlorid  is  yel- 
lowish in  color  and  nearly  insoluble  in  cold  water. 
Boiling  water,  however,  converts  it  into  auric  chlorid 
and  metallic  gold.  Its  composition  is  AuCl ;  atomic 
weight,  232.1. 

Auric  chlorid  is  the  most  important  of  the  com- 
pounds of  gold,  and  is  the  source  from  which  most 
of  the  preparations  of  gold  used  in  the  arts  are  ob- 
tained. There  are  iodids  of  gold  resembling  the 
chlorids  in  many  respects.  Berzelius  also  described 
an  aurous  sulfid.     These  are,  however,  not  important. 

Purple  of  Cassius,  named  for  the  discoverer,  M. 
Cassius,  is  employed  by  manufacturers  of  porcelain 
teeth  in  obtaining  the  gum  color,  and  in  the  industrial 
arts  for  imparting  a  red  color  to  glass  and  porcelain. 
It  is  a  compound  of  gold,  tin,  and  oxygen,  which  are 
believed  to  be  grouped  according  to  the  formula — 

Au,O.SnO..„  SnO,  SnO._,-f  4H,0.* 

It  may  be  prepared  in  the  humid  way  by  adding 
stannous  chlorid  (SnCl.,)  to  a  mixture  of  stannic  chlorid 
(SnClJ  and  trichlorid  of  gold.  Seven  parts  of  gold 
are  dissolved  in  aqua  regia  and  mixed  with  two  parts 
of  tin,  also  dissolved  in  aqua  regia.     This  solution  is 

*  Bloxam's  Chemistry,  Organic  and  Inorganic. 


150  DENTAL     METALLURGY. 

largely  diluted  with  water,  and  a  weak  solution  of  one 
part  of  tin  in  hydrochloric  acid  is  added,  drop  by 
drop,  until  a  fine  purple  color  is  produced.  The  pur- 
ple of  Cassius,  in  a  state  of  fine  division,  remains  for 
a  time  suspended  in  the  water,  but  finally  subsides  as 
a  purple  powder.  The  fresh  precipitate  dissolves  in 
ammonia,  and  exposure  to  light  decomposes  the  pur- 
ple solution,  during  which  process  its  hue  changes  to 
blue,  and  it  finally  becomes  colorless,  and  metallic 
gold  is  precipitated,  the  binoxid  of  tin  being  left  in 
solution. 

The  dry  method*  is  the  one  now  employed  by 
manufacturers  of  porcelain  teeth  in  the  preparation  of 
gum-enamel.  Two  hundred  and  forty  grains  of  pure 
silver,  twenty-four  grains  of  pure  gold,  and  seventeen 
and  a  half  grains  of  pure  tin  are  placed  in  a  crucible, 
with  sufficient  borax  to  cover  the  mass,  and  melted. 
In  order  to  insure  a  thorough  mixture  of  the  different 
metals,  the  melted  mass  should  be  poured  from  a 
height  into  a  vessel  of  cold  water,  and  this  process  of 
granulation  should  be  repeated  at  least  three  times, 
but  at  every  melting  the  alloy  should  be  well  covered 
with  borax  to  prevent  loss  of  the  tin  by  oxidation. 
The  vessel  into  which  the  melted  mass  is  poured 
should  not  be  a  metallic  one. 

The  component  parts  of  the  alloy  having  now  been 
thoroughly  incorporated,  the  next  step  is  to  collect 
the  granulated  mass  and  separate  from  it  any  adherent 
particles  of  glass  of  borax.     The  metal  is  then  put 

The  dry  method  of  preparing  purple  of  Cassius  and  the  process  of 
manufacturing  the  gum-enamel  were  imparted  to  the  author  by  the  late 
Professor  Wildman,  to  whom  is  due  the  credit  of  having  brought  the  pre- 
paration of  bodies  and  enamels  to  their  present  high  state  of  excellence. 


GOLD.  151 

into  a  glass  or  porcelain  evaporating-dish  (the  Berlin 
porcelain  is  the  best),  and  sufficient  chemically  pure 
nitric  acid  is  added  to  cover  the  metal.  The  dish  is 
now  placed  over  a  sand-bath,  and  gentle  heat  applied 
and  continued  until  chemical  action  ceases.  If  at  this 
point  it  is  found  that  all  the  metallic  particles  are  dis- 
solved, the  dish  may  be  removed  from  the  bath. 
Should  any  solid  particles  be  found  in  the  solution,  a 
little  more  nitric  acid  must  be  added,  and  the  opera- 
tion continued  until  all  are  dissolved.  The  silver 
having  been  entirely  dissolved  by  the  nitric  acid,  the 
solution  should  be  poured  off,  and  the  remaining  oxid 
carefully  washed  until  the  last  trace  of  silver  is  re- 
moved. After  several  washings  with  a  large  quantity 
of  pure  warm  water,  the  latter  should  finally  be  tested 
with  a  clear  solution  of  common  salt,  and  if  it  remains 
clear,  without  show  of  milkiness,  the  silver  is  all  re- 
moved. When  the  oxid  is  sufficiently  washed,  the 
purple  of  Cassius  should  be  dried  by  gently  heating, 
after  which  it  is  ready  to  be  incorporated  with  the 
silicious  materials. 

The  process  of  making  gum-enamel  is  divided  into 
three  stages  :  first,  the  preparation  of  the  oxid  ;  sec- 
ond, fritting,  or  by  the  aid  of  heat  uniting  the  metallic 
oxid  with  the  silicious  base  ;  and,  third,  diluting  the 
frit  so  as  to  form  the  desired  shade.  The  first  we 
have  already  described.  The  frit  is  formed  by  mixing 
eight  grains  of  the  metallic  oxid  (purple  of  Cassius) 
with  seven  hundred  grains  of  feldspar,  and  one  hun- 
dred and  seventy-five  grains  of  a  flux  composed  of — 

Pure  quartz      .         ....     4  ounces  ; 
Glass  of  borax                                    .     1  ounce  ; 
Sal  tartar 1  ounce  ; 


152  DENTAL     METALLURGY. 

fused  into  a  glass  and  ground  fine.  The  oxid  is  placed 
in  a  smooth  Wedgwood  mortar  and  ground  separately 
as  fine  as  it  is  possible  to  get  it.  The  flux  is  then 
added  in  small  quantities,  and  the  levigation  con- 
tinued, after  which  the  feldspar  may  be  added  and 
treated  similarly.  It  is  of  the  highest  importance 
that  the  mass  be  reduced  to  the  utmost  degree  of  fine- 
ness, and  an  expert  workman  will  spend  six  or  eight 
hours  at  least  in  levigating  the  quantity  given  in  the 
formula.  While  the  mass  is  being  ground  in  the 
mortar,  foreign  substances,  such  as  small  particles  of 
wood,  etc.,  must  be  carefully  excluded  ;  otherwise, 
during  the  vitrifying  process,  these  will  be  converted 
into  carbon,  which  will  be  sure  to  reduce  a  portion  of 
the  gold  in  fine  metallic  globules  distributed  through- 
out the  mass. 

The  vitrifying  or  fritting  process  consists  in  pack- 
ing the  mass,  after  the  most  thorough  levigation,  in 
the  whitest  sand  crucible  that  can  be  obtained.  (Dark- 
colored  crucibles  are  liable  to  injure  the  frit  by  con- 
tamination with  iron.)  This  must  be  provided  with 
an  accurately  fitting  cover  made  of  the  same  material, 
or  a  suitable  top  may  be  formed  of  a  piece  of  slide 
such  as  is  used  in  burning  continuous-gum  work. 
Before  placing  the  frit  in  the  crucible,  the  interior  sur- 
face of  the  latter  should  receive  a  thin  coating  of  very 
fine  quartz,  made  into  a  paste  with  water,  to  prevent 
the  frit  from  adhering  to  it  during  fusion.  The  frit  in 
a  dry  state  is  then  packed  in,  and  the  cover  tightly 
luted  to  its  place  with  kaolin.  The  crucible  is  then  to 
be  buried  in  a  strong  anthracite  coal  fire,  and  to  remain 
there  until  the  contents  are  fused.  The  time  required 
to  do  this  will  depend  upon  the  size  of  the  crucible 


GOLD.  153 

and  the  intensity  of  the  heat.  Any  ordinary  coal- 
stove  provided  with  a  good  draught  will  answer  ;  but 
the  fuel  must  be  packed  around  and  over  the  crucible, 
and  the  heat  carried  to  the  highest  attainable  point. 
Usually  about  two  hours  will  be  required  to  thoroughly 
fuse  the  mass,  after  which  it  is  removed  from  the  fire 
and  permitted  to  cool. 

The  vitrified  mass  is  removed  from  the  crucible  by 
breaking  the  latter.  Every  particle  of  adhering  quartz 
or  portions  of  the  crucible  should  be  cleared  from  the 
surface.  It  is  then  pulverized  to  a  fineness  which  will 
allow  it  to  pass  through  a  No.  10  bolting-cloth  sieve, 
and  is  ready  for  the  third  stage  in  the  preparation  of 
gum-enamel,  which  consists  of  diluting  the  frit  with 
the  proper  amount  of  feldspar.  As  the  strength  of 
the  coloring-pigment  varies  according  to  the  degree 
of  fineness  attained  during  the  levigation,  it  is  usually 
necessary  to  make  several  tests  in  order  to  arrive  at 
the  desired  shade.  This  is  accomplished  by  mixing 
separately  several  different  lots  in  the  following  pro- 
portions : 


Gum  frit 


1  part ; 


Feldspar .2  parts. 

Gum  frit       ".....     1  part ; 
Feldspar .3  parts. 

Gum  frit 1  part ; 

Feldspar 4  parts. 

These  are  applied  to  marked  pieces  of  porcelain  body 
and  fused  in  the  usual  way,  the  result  determining  the 
proportions  necessary  to  produce  the  desired  shade. 

There  is  a  compound  known  as  "  silicate  of  gold," 
used  in  ceramic  dentistry  to  impart  a  life-like  yellow- 
tint  to  porcelain  teeth.     This  is  prepared  by  grinding 

11 


154  DENTAL   METALLURGY. 

together  in  a  Wedgwood  mortar  one  hundred  and 
twenty  grains  of  coarse  feldspar,  ten  grains  of  gold 
foil,  and  eight  grains  of  flux.*  These  are  ground 
until  the  gold  is  entirely  cut  up,  when  the  mass  is  made 
into  a  ball  and  placed  on  a  slide  and  fused,  after  which 
it  is  again  ground  fine  and  is  then  ready  for  use. 

Hyposulfite  of  gold  and  soda,  the  sel  d?  or  of  the 
photographers,  is  a  double  salt  formed  by  adding  a 
solution  of  one  part  of  trichlorid  of  gold  to  a  solution 
of  three  parts  of  hyposulfite  of  soda.  Alcohol,  in  which 
the  double  salt  is  soluble,  is  then  added.  The  forma- 
tion of  this  compound  may  be  explained  by  the  equa- 
tion,  8(N02S203)  +  2AuC13  =  Au2S203,  3(Na2S203)  + 

6NaCl+2(Na2S406)- 

Fulminating   Gold. — When  ammonia  is  added  to 

trichlorid  of  gold,  a  buff-colored  precipitate  results, 

which    explodes  violently   when  gently  heated.     Its 

exact  composition  is  not  well  established. 

Discrimination  of  Gold. — Protochlorid  of  tin  is  a 
characteristic  test  for  gold,  affording  a  purple-brown 
precipitate.  The  smallest  portion  of  gold  dissolved 
in  a  large  quantity  of  water  may  be  detected  by  the 
addition  of  a  few  drops  of  this  reagent.  Thus,  a  pale 
brown  precipitate  may  be  obtained  in  a  pint  of  water 
containing  but  one-fiftieth  of  a  grain  of  gold. 

Ferrous  sulfate  is  also  a  delicate  test  for  the  presence 

*  White  bottle-glass,  which  does  not  contain  lead  or  iron,  may  be  used 
to  reduce  the  fusing-point  of  enamels,  but,  owing  to  the  uncertainty  of  the 
composition  of  glass,  most  of  the  manufacturers  of  porcelain  teeth  make  a 
fine  glass,  for  this  purpose,  of  the  following  proportions :  Pure  quartz, 
four  ounces  ;  glass  of  borax,  one  ounce ;  sal  tartar,  one  ounce.  These  are 
first  ground  separately,  then  thoroughly  mixed,  and  placed  in  a  white 
crucible  provided  with  a  cover,  which  must  be  tightly  luted,  and  then 
thoroughly  fused  in  the  fire.  If  perfectly  pure  materials  are  used  the 
result  will  be  an  exceedingly  brilliant,  colorless,  and  transparent  glass. 


GOLD.  155 

of  gold,  and  will  detect  the  merest  trace  of  it.  By 
this  reagent  the  gold  is  thrown  down  in  the  form  of  a 
brown  powder,  which,  after  washing,  drying,  and  heat- 
ing to  redness,  yields  the  metal  in  a  finely  divided 
state. 

Sulfuretted  hydrogen  (H.,S)  added  to  a  solution  of 
trichlorid  of  gold  affords  a  brown  precipitate  of  auric 
sulfid.  .  Nitrate  of  mercury  also  precipitates  from  solu- 
tion a  brown  powder,  which  after  heating  yields  finely 
divided  gold.  Finely  divided  gold,  suspended  in 
water,  imparts  a  violet  or  red  color  to  it.  Colored 
fluids  containing  minute  particles  of  gold  in  a  state  of 
suspension  may  be  obtained  by  the  action  of  phos- 
phorus dissolved  in  ether  upon  a  very  weak  solution 
of  gold  in  aqua  regia.  After  standing  for  a  long  time 
the  fine  particles  of  gold  are  deposited,  having  the 
same  tint  as  that  which  they  previously  exhibited  when 
suspended  in  the  liquid.  The  blue  particles,  being 
less  minute,  are  soonest  deposited,  but  the  red  parti- 
cles require  many  months  to  settle  down.  The  one- 
hundredth  of  a  grain  of  gold  is  capable  of  imparting 
a  deep  rose-color  to  a  cubic  inch  of  fluid,  and  the  dif- 
ferent colors  thus  produced  are  taken  advantage  of  in 
painting  upon  porcelain,  a  beautiful  ruby-red  color 
being  the  result  of  the  pigment  thus  obtained. 

Assays  of  Gold  Ores,  Quartz,  etc. — The  specimens 
of  ores  should  first  be  heated  to  redness,  and  then 
thrown  into  cold  water  to  facilitate  powdering,  which 
may  be  accomplished  in  an  ordinary  Wedgwood 
mortar.  Several  lots  of  three  hundred  grains  each 
should  be  weighed  and  examined  separately,  and  the 
assays  made  from  these  averaged  for  the  result.  To 
the  separate  portions  of  powdered  ore  equal  weights 


I56  DENTAL   METALLURGY. 

of  litharge,  half  their  weights  of  sodic  carbonate,  and 
about  half  of  powdered  charcoal,  are  added  and  thor- 
oughly mixed  with  the  ore.  Each  portion  is  then 
placed  in  a  crucible,  a  little  borax  sprinkled  over  the 
top,  and  it  is  ready  for  heating  in  a  suitable  blast-  or 
wind-furnace.  The  heat  at  first  should  be  gradual,  so 
that  the  active  effervescence  caused  by  the  escape  of 
carbonic  acid  from  the  soda  salt  may  not  force  por- 
tions of  the  mixture  from  the  crucible.  After  a  short 
time,  however,  the  danger  from  this  cause  having 
passed,  the  heat  may  be  carried  to  bright  redness,  or 
until  the  whole  has  fused.  An  ingot-mold  with  two 
apertures  has  been  recommended  for  the  reception  of 
the  fused  mixture,  which  at  this  point  is  ready  for 
pouring,  the  slag  being  turned  into  one  concavity  and 
the  reduced  metal  into  the  other.  The  button  will  be 
found  to  consist  of  lead  and  gold,  the  former  reduced 
from  the  litharge  and  the  latter  from  the  auriferous 
ore.     These  are  to  be  separated  by  cupellation. 

If  the  ore  contains  much  iron  pyrites,  or  is  of  the 
nature  of  '  ■  sweep' '  (the  name  given  to  residues  which 
accumulate  in  the  dental  laboratory  and  other  places 
where  gold  is  worked),  it  will  be  necessary  to  roast  it 
in  a  shallow  fire-clay  dish  placed  in  a  muffle  ;  and,  in 
the  case  of  pyrites  containing  about  seven  penny- 
weights to  the  ton,  the  operation  should  be  conducted 
with  one  thousand  grains.  The  roasted  ore  is  then 
fused  with  a  mixture  consisting  of  red  lead,  one  thou- 
sand grains  ;  sodic  carbonate,  six  hundred  grains  ; 
powdered  charcoal,  forty  grains,  and  borax,  five  hun- 
dred grains.  The  mixture  is  introduced  into  a  clay 
crucible,  which  it  should  half  fill,  and  is  fused  in  an 
air-furnace.     The   button  of   reduced   lead   may  be 


GOLD.  157 

removed  either  by  pouring  the  contents  of  the  cru- 
cible into  a  mold,  or  by  breaking  the  crucible  when 
cold. 

Assay  by  Scorification. — Scorirication  resembles 
cupellation,*  but  the  oxid  of  lead  produced  in  the 
operation,  instead  of  sinking  into  a  porous  cup,  is  held 
in  a  flat  saucer  of  fire-clay,  and  dissolves  the  earthy 
constituents  of  the  ore,  leaving  the  precious  metal  to 
pass  into  another  portion  of  lead,  which  remains  in 
the  metallic  state.  About  two  hundred  grains  of  the 
roasted  ore  are  placed  in  the  scorifier,  and  intimately 
mixed  with  five  hundred  grains  of  granulated  and  fifty 
erains  of  borax  lead.  The  contents  of  the  scorifier 
are  fused  in  a  muffle.  Air  is  admitted  to  oxidize  the 
greater  portion  of  the  lead.  At  the  conclusion  of  the 
operation  the  litharge  should  be  perfectly  fluid  and 
cover  the  molten  lead.  The  slag  may  be  freed  from 
particles  of  precious  metal  by  the  addition,  at  the 
conclusion  of  the  operation,  of  a  small  quantity  of 
powdered  anthracite,  which  reduces  a  portion  of  the 
litharge  to  metallic  globules,  which  fall  through  the 
slag  and  unite  with  the  lead  button.  The  gold  is  then 
separated  by  cupellation,  and  the  silver,  with  which  it 
is  almost  always  associated,  by  parting  with  nitric  acid. 

Assaying. — This  term  refers  to  the  quantitative 
estimation  of  one  constituent  of  an  alloy  or  mineral, 
and  is  accomplished  by  cupellation  when  the  alloying 
metal  is  copper,  and  ' '  parting' '  when  the  debasing 
metal  consists  of  silver.  Usually  both  operations  are 
necessary.  From  five  to  sixteen  grains  of  the  gold 
are  wrapped  in  sheet  lead,  with  pure  silver  equal  to 
two  and  a  half  times  the  quantity  of  gold  supposed 

*  See  page  158. 


158  DENTAL    METALLURGY. 

to  be  present.  The  weight  of  lead  employed  where 
the  assay  is  standard  gold*  is  8  to  1 ,  and  the  ratio  of 
the  weight  of  lead  to  the  weight  of  copper  assumed 
to  be  present  is  100. 1.  The  assay  is  now  to  be  treated 
by  cupellation,  a  process  which  is  thus  briefly  and 
clearly  described  by  Mr.  W.  Crookes  : 

1 '  The  gold  alloy  is  fused  with  a  quantity  of  lead 
and  a  little  silver,  if  silver  is  already  present.  The 
resulting  alloy,  which  is  called  the  'lead  button,'  is 
then  submitted  to  fusion  on  a  very  porous  support, 
made  of  bone-ash  and  called  a  '  cupel.'  The  fusion 
is  effected  in  a  current  of  air,  which  oxidizes  the  lead. 
The  heat  is  sufficient  to  keep  the  oxid  of  lead  fused. 
The  porous  cupel  has  the  property  of  absorbing 
melted  oxid  of  lead  without  taking  up  any  of  the 
metallic  globules,  exactly  in  the  same  way  that  blotting- 
paper  will  absorb  water  while  it  will  not  touch  a  globule 
of  mercury.  The  heat  being  continued,  and  the  cur- 
rent of  airf  always  passing  over  the  surface  of  the 
melted  lead  button,  and  the  oxid  of  lead  or  litharge 
being  sucked  up  by  the  cupel  as  fast  as  it  is  formed, 
the  metallic  globule  rapidly  diminishes  in  size  until  at 
last  all  the  lead  has  been  got  rid  of.  Now,  if  this 
were  the  only  action,  little  good  would  have  been 
gained,  for  we  should  have  put  lead  into  the  gold  alloy 
and  taken  it  out  again.  But  another  action  goes  on 
while  the  lead  is  oxidizing  in  the  current  of  air.  Other 
metals,  except  the  silver  and  gold,  also  oxidize,  and 

*  22-carat,  or  coin. 

t  The  process  of  cupellation  is  generally  performed  in  a  furnace  pro- 
vided with  a  muffle  for  the  reception  of  the  cupels,  and  arranged  so  as  to 
admit  of  a  current  of  air  over  the  fused  button.  The  lead  used  in  cupella- 
tion should  be  of  absolute  purity  ;  otherwise,  as  lead  is  always  liable  to 
contain  silver,  the  latter  would  necessarily  combine  with  the  assay  and 
vitiate  the  accuracy  of  the  result. 


GOLD.  159 

are  carried  by  the  melted  litharge  into  the  cupel.  If 
the  lead  is,  therefore,  rightly  proportioned  to  the 
standard  of  alloy,  the  resulting  button  will  consist  of 
only  gold  and  silver,  and  these  are  separated  by  the 
operation  of  parting,  which  consists  in  boiling  the 
alloy  (after  rolling  it  into  a  thin  plate)  in  strong  nitric 
acid,  which  dissolves  the  silver  and  leaves  the  gold  as 
a  coherent  sponge.  "* 

As  the  accuracy  of  the  result  of  an  assay  is  liable  to 
be  influenced,  either  by  retention  of  silver  or  copper, 
or  by  loss  of  gold  by  volatilization  in  the  muffle,  solu- 
tion in  the  acid,  or  retention  in  the  cupel,  it  is  neces- 
sary to  employ  "check  assays,"  made  on  pure  gold, 
with  which  the  alloy  assay  is  weighed  in  comparison  ; 
and,  as  will  be  seen,  the  weight  of  gold  indicated  by 
the  balance  is  liable  to  be  either  greater  or  less  than 
the  quantity  originally  present  in  the  alloy.  The 
following  formulaf  will  serve  to  show  the  correction 
to  be  applied  : 

Let  1000  be  the  weight  of  alloy  originally  taken. 

p,  the  weight  of  the  piece  of  gold  finally  obtained. 

-r,  the  actual  amount  of  gold  in  the  alloy  expressed  in 
thousandths. 

a,  the  weight  of  gold  (supposed  to  be  absolutely  pure) 
taken  as  a  check,  which  approximately  equals  x. 

6,  the  loss  or  gain  of  weight  experienced  by  a  during  the 
process  of  assay,  expressed  in  thousandths. 

k,  the  variation  of  "check  gold"  from  absolute  purity, 
expressed  in  thousandths. 

Then  the  actual  amount  of  fine  gold  in  the  check-piece 
=a(i — *  ),  and  .r,  the  corrected  weight  of  the  assay,  will 
=p —  ak+b  ;  b  being  added  or  subtracted  according  as  it  is 
a  loss  or  gain. 

*  See  "  Quartation."  f  Annual  Report,  Mint  of  England. 


i6o 


DENTAL   METALLURGY. 


If  a  be  assumed  to  be  equal  to  x,  this  equation  becomes 

x—  j  a.    k 

Example. — Let^  =  901.  i  thousandths 

a  =  920.0 

6  =     0.3  "  gain  in  weight. 

£  =     0.1 
Then  by  the  first  formula 

-V r\r\T    T   9  2  0  +  0.1   n    ■> 

X 9OI.  I  TO  Off  °'3 

For,  as  £  is  a  gain  in  weight,  it  must  be  deducted.     Hence, 

.r  — 901.1  — 0.092  —  0.3. 

=  900.708. 

And  by  the  second  formula 

901. 1  —  0.3 

X~    T  j_  _o^_i_ 
1   1   1000 

=  900.708. 
Gold  Leaf  and  Foil.— -Gold  designed  for  manufac- 
ture into  leaf  is  variously  alloyed,  according  to  the 
color  required.  The  following  is  a  list  showing  the 
proportions  of  alloy  per  ounce  required  to  produce 
certain  tints  : 


Color  of  Leaf. 

Proportions 
of  Gold. 

Proportions 
of  Silver. 

Proportions 
of  Copper. 

Grains. 

Grains. 

Grains. 

Red. 

456.460 

— 

20.24 

Pale  Red  . 

464. 

— 

16. 

Extra  Deep  Red 

456. 

12 

12. 

Deep  Red 

444. 

24 

12. 

Citron 

440. 

30 

IO. 

Yellow 

408. 

72 

— 

Pale  Yellow     . 

384. 

96 

— 

Lemon 

360. 

I20 

— 

Green  or  Pale  . 

312. 

168 

White 

240. 

24O 

. 

GOLD.  .  l6l 

For  rilling  teeth  nearly  pure  gold  in  the  form  of  foil 
is  used.  It  is  generally  prepared  by  beating,  but  some 
of  the  heavier  numbers  are  produced  by  rolling.  There 
are  two  varieties,  cohesive  and  non-cohesive,  exten- 
sively used  in  the  United  States  at  the  present  time, 
the  methods  of  manipulating  which  are  widely  differ- 
ent. In  the  former  the  characteristic  quality  of  co- 
hesiveness,  which  is  greatly  diminished  by  compression 
of  the  fibers  in  beating,  is  restored  by  heating  to  red- 
ness, and  this  is  best  effected  by  placing  the  foil  upon 
a  sheet  of  mica,  which  is  held  over  a  spirit-lamp.  The 
habit  common  among  dentists,  of  taking  up  the  foil 
on  the  point  of  the  plugger  and  passing  it  through 
the  flame  of  a  spirit-lamp,  is  not  productive  of  the 
best  results,  the  gold  being  made  harsh  by  contact 
with  the  flame,  whereas  it  should,  in  addition  to  co- 
hesiveness,  possess  at  least  some  of  the  kid-like  soft- 
ness of  the  non-cohesive  variety.  Some  of  the  manu- 
facturers of  gold  foil  for  dental  purposes  produce  a 
4 '  soft' '  variety,  in  which  cohesiveness  cannot  be  de- 
veloped by  heating.  This  quality  may  be  attained  by 
alloying,  or  by  depositing  carbon  upon  the  surface,  as 
the  latter  cannot  be  driven  off  by  heating. 

The  statement  has  been  made  by  one  of  the  most 
experienced  manufacturers  of  dental  foils  in  this 
country,  that  he  makes  the  two  varieties  of  cohesive 
and  non-cohesive  foils  from  the  same  ingot.  As  his 
non-cohesive  or  "soft"  foil  is  unsurpassed  in  the 
qualities  which  are  desirable  for  such  a  foil,  it  seems 
proper  to  assume  therefore  that  the  development  of 
these  qualities  is  due  either  to  some  treatment  of  the 
surface  or  to  mechanical  management  during  lamina- 
tion, and  not  to  alloying. 


l62  DENTAL     METALLURGY. 

The  manufacture  of  non-cohesive  gold  foil  properly 
so  called  is  not,  it  would  seem,  understood  by  every 
gold-beater .  Some  of  them  prepare  only  cohesive  foil, 
while  others  offer  for  sale  a  so-called  non-cohesive  foil, 
which  is  really  nothing  more  than  an  unannealed  sam- 
ple of  the  cohesive  type,  and  which  is  destitute  of  the 
peculiar  ' '  kid-like  softness' '  and  toughness  which 
permits  it  to  be  carried  forward  by  the  plugging  instru- 
ment into  deep  cavities  without  fracture.  The  extent 
to  which  the  characteristic  qualities  of  cohesiveness, 
etc.,  may  be  modified  or  entirely  lost  by  the  absorp- 
tion of  gases  has  yet  to  be  fully  studied.  In  1866 
Graham  demonstrated  that  gold  is  capable  of  occlud- 
ing 0.48  of  its  volume  of  hydrogen,  and  0.20  of  its 
volume  of  nitrogen.  Varrentrapp  has  also  pointed  out 
that  ' '  cornets' '  from  the  assay  of  gold  may  retain  gas 
if  they  are  not  strongly  heated. 

Prof.  G.  V.  Black  deserves  credit  for  his  researches 
published  in  the  Dental  Cosmos,  vol.  xviii,  p.  138, 
showing  the  influence  of  gases  and  moisture  on  the 
cohesiveness  of  gold  foil. 

The  corrugated  gold  introduced  about  fifteen  years 
ago  belongs  to  this  class  of  foils.  It  is  prepared  by  plac- 
ing the  vsheets  of  gold  between  leaves  of  a  particular 
kind  of  unsized  paper,  and  tightly  packing  it  in  iron 
boxes,  which  are  exposed  to  a  temperature  sufficiently 
high  to  carbonize  the  paper.  These  are  then  allowed 
to  cool,  and  on  opening  them  the  gold  is  found  to  be 
exceedingly  soft  and  non- cohesive,  and  to  present  a 
peculiar  corrugated  condition  of  surface,  while  it  is 
incapable  of  being  rendered  cohesive  by  annealing. 

Non-cohesive  foil  is  used  in  the  form  of  cylinders, 
made  by  rolling  a  ribbon  or  strip  of  foil,  or  as  pellets, 


GOLD.  163 

mats,  or  ropes.  These  are  introduced  into  the  cavity 
by  means  of  plugging  instruments  with  or  without 
serrations,  the  force  employed  being  for  the  most 
part  hand-pressure,  though  the  hand-mallet  is  much 
used  by  many  operators  as  a  means  of  impacting  non- 
cohesive  foil.  The  advantages  claimed  for  non-cohesive 
foil  are  that  less  time  is  consumed  in  its  introduction, 
and  that  in  consequence  of  the  greater  softness  which 
it  possesses  it  is  capable  of  being  more  thoroughly 
burnished  to  the  edges  of  the  cavity. 

When  non-cohesive  foil  is  used,  union  does  not  take 
place  between  the  particles  of  gold  introduced  into  the 
cavity.  They  are  simply  made  to  adhere  mechanically 
by  wedging  the  mats,  cylinders,  or  pellets,  as  the  case 
may  be,  one  against  the  other,  into  a  cavity  properly 
prepared  to  retain  them.  On  the  other  hand,  between 
particles  of  gold  wherein  the  characteristic  quality  of 
cohesiveness  remains  unimpaired,  that  property  mani- 
fests itself  whenever  two  pieces  are  brought  in  contact, 
and  if  cohesion  be  facilitated  by  the  application  of 
sufficient  force,  homogeneity  results.  Hence,  the 
methods  of  operating  with  the  two  kinds  of  foil  must 
necessarily  differ.  Non-cohesive  foil  is  introduced  in 
pieces  of  more  or  less  bulk  ;  the  cohesive  variety  is 
introduced  in  much  smaller  proportions,  each  piece 
being  carefully  welded  to  the  others  by  means  of  the 
electro-magnetic  mallet,  the  hand-mallet,  the  "auto- 
matic" plugger,  or  by  hand-pressure. 

The  method  of  preparing  cohesive  foil  is  thus  de- 
scribed by  the  late  Dr.  M.  H.  Webb,  of  Lancaster,  Pa. : 
"A  half-leaf  of  No.  4  gold  foil  for  small,  a  whole  sheet 
for  medium,  and  two  leaves  for  large  fillings,  should 
be  taken  from  the  book  by  means  of  the  foil-carrier  or 


164  DENTAL    METALLURGY. 

spatula,  and  placed  upon  a  piece  of  spunk  covered 
with  white  kid.  The  foil  should  be  folded  with  an 
ivory  spatula  into  a  tape-like  form,  eight  or  ten  lines 
in  width  respectively,  which  is  then  cut  across  into 
small  pieces  about  one-twelfth  of  an  inch  in  width. 
Heavy  foils,  ranging  from  No.  30  to  No.  60,  may  be 
advantageously  employed  in  extensive  contour  opera- 
tions. The  ordinary  light  numbers,  however,  when 
prepared  in  the  manner  described,  can  more  easily  be 
impacted  into  small  cavities,  fissures,  and  grooves." 

For  cohesive  gold  the  cavity  is  carefully  prepared, 
the  edges  of  enamel  being  smoothly  and  evenly  fin- 
ished. A  groove  or  undercut  is  then  made  toward  the 
cutting-edge  and  toward  the  cervical  wall,  and  a  start- 
ing-point drilled  in  the  dentine  toward  the  palatal  edge 
for  the  purpose  of  anchorage.  Into  this  the  gold  is 
carried  by  means  of  hand-pressure,  and  by  the  same 
means  worked  into  the  grooves  or  undercuts.  The 
point  in  which  to  start  the  filling  should  be  only  suffi- 
ciently deep  to  retain  the  small  pieces  of  gold  first 
introduced  while  others  are  being  built  upon  them. 
When  the  first  pieces  are  firmly  fixed,  the  electro- 
magnetic mallet  may  be  employed  to  the  end.  If  the 
cavity  be  small,  the  gold  should  be  prepared  from  a 
half-leaf  of  No.  4  foil,  so  folded  as  to  be  equivalent  to 
No.  12  or  No.  16,  and  then  cut  into  strips  about  one- 
twenty-fourth  of  an  inch  in  width. 

Although  much  diversity  of  opinion  exists  among 
dentists  regarding  the  relative  value  of  cohesive  and 
non-  cohesive  foils,  it  may  with  safety  be  stated  that, 
skillfully  directed,  either  is  capable  of  affording  good 
results.  But,  like  all  other  filling-materials,  each  has 
its  proper  place.     Thus,  in  a  majority  of  crown-cavi- 


GOLD.  165 

ties,  it  would  be  a  sheer  waste  of  time  to  fill  exclus- 
ively with  cohesive  foil;  while,  on  the  other  hand,  there 
are  many  approximal  cavities  in  which  the  walls  have 
been  rendered  so  thin  by  the  progress  of  decay  that 
probably  the  electro-magnetic  mallet  alone  could  be 
relied  on  to  thoroughly  impact  the  gold  to  walls  and 
periphery.  In  such  cases  the  cohesive  foil  should  be 
folded  of  No.  4,  and  cut  into  narrow  strips  as  above 
described,  the  main  object  being  to  avoid  the  applica- 
tion of  much  force,  such  as  would  be  required  in  the 
consolidation  of  large  mats  or  cylinders  of  non-cohe- 
sive gold.  Indeed,  it  may  be  stated  that  one  of  the 
greatest  advantages  in  filling  with  cohesive  foil  by  the 
electro-magnetic  mallet  is  the  thoroughness  and  safety 
with  which  the  gold  may  be  packed  against  very  frail 
walls. 

The  merits  of  the  cohesive  and  non-cohesive  forms 
of  foil  are  by  their  respective  advocates  often  unfairly 
presented,  but  the  value  of  each  may  be  stated  as  fol- 
lows :  Cohesive  foil  in  very  small  pieces  is,  with  the 
electric  plugger,  capable  of  being  brought  into  perfect 
apposition  with  the  most  delicate  walls  of  enamel  with 
comparatively  little  danger  of  fracture  ;  non-cohesive 
foil,  in  much  larger  masses,  may  with  less  expenditure 
of  time  be  introduced  into  cavities  where  the  walls  are 
sufficiently  strong  to  withstand  the  force  required  to 
consolidate  the  mats  or  cylinders,  and,  so  far  as  the 
preservation  of  the  tooth  is  concerned,  affords  equally 
good  results. 

Gold- Beating. — In  all  probability  the  art  of  gold- 
beating  originated  among  Oriental  communities,  with 
whom  the  love  of  gold  ornaments  has  always  been  a 
distinguishing  characteristic.      The    art    is    of  great 


1 66  DENTAL     METALLURGY. 

antiquity,  and  is  referred  to  by  Homer  and  Pliny.  On 
the  coffins  of  Theban  mummies  specimens  of  leaf-gild- 
ing were  met  with,  where  the  gold  in  a  very  thin  state 
resembled  modern  gilding.  It  is  stated  that  the  Incas 
of  Peru  did  not  understand  the  art  of  gold-beating 
beyond  the  preparation  of  sheets  or  plates,  which  they 
nailed  on  the  walls  of  their  temples. 

The  process  of  beating  gold  is  conducted  in  the 
following  manner  :  The  metal  is  first  alloyed  accord- 
ing to  the  color  desired,  and,  in  order  to  improve  its 
malleability,  it  is  melted  at  a  higher  temperature  than 
is  necessary  for  mere  fusion.  It  is  then  cast  into  an 
ineot  and  rolled  into  a  ribbon  of  a  half-inch  in  width 
and  ten  feet  in  length  to  the  ounce.  After  this  it  is 
annealed  and  cut  into  pieces  of  about  six  and  a  half 
grains  each,  and  placed  between  the  leaves  of  a 
"  cutch,"  which  is  about  half  an  inch  thick  and  three 
and  a  half  inches  square,  containing  about  one  hun- 
dred and  eighty  leaves  of  a  tough  paper  manufactured 
in  France.  Fine  vellum  was  formerly  much  used  for 
this  purpose,  and  it  is  yet  often  interleaved^  in  the  pro- 
portion of  about  one  of  vellum  to  six  of  paper.  The 
hammer  employed  by  gold-beaters  weighs  about  seven- 
teen pounds,  and  rebounds,  by  the  elasticity  of  the 
skin,  to  such  an  extent  that  each  stroke  involves  but 
little  labor.  It  requires  about  twenty  minutes'  beat- 
ing to  spread  the  gold  to  the  size  of  the  cutch,  and  if 
it  is  intended  for  filling  teeth  it  is  carried  no  further 
than  the  cutch  stage.  If,  however,  it  is  to  be  still 
further  attenuated,  each  leaf  is  taken  from  the  cutch 
and  cut  into  four  pieces,  when  it  is  put  between  the 
skins  of  a  "shoder,"  four  and  a  half  inches  square 
and  three-quarters  of  an  inch  thick,  containing  about 


GOLD.  167 

seven  hundred  and  twenty  skins.  The  shoder  requires 
about  two  hours'  beating  with  a  nine-pound  hammer. 
As  the  gold  will  spread  unequally,  the  shoder  is  beaten 
upon  after  the  larger  leaves  have  reached  the  edges, 
the  effect  of  which  is  that  the  margins  of  larger  leaves 
come  out  of  the  edges  in  a  state  of  dust.  This  allows 
time  for  the  smaller  leaves  to  reach  the  full  size  of  the 
shoder,  by  which  a  general  evenness  in  the  size  of  the 
leaves  is  obtained.  Each  leaf  is  again  cut  into  four 
pieces  and  placed  between  the  leaves  of  a  "  mold,"  — 
an  appliance  composed  of  about  nine  hundred  and 
fifty  of  the  finest  gold-beaters'  skins.  Its  dimensions 
are  five  inches  square  by  three-fourths  of  an  inch 
thick.  The  management  of  the  gold  in  the  "  mold" 
is  the  last  and  most  difficult  stage  in  the  process  of 
gold-beating,  and  the  fineness  of  the  skin  and  judg- 
ment of  the  workman  will  greatly  influence  the  final 
result. 

The  process  of  lamination  may  be  thus  described  : 
During  the  first  hour  the  blows  of  the  hammer  are 
directed  principally  upon  the  center  of  the  mold,  by 
which  means  the  edges  of  the  leaves  are  made  to 
crack,  but  they  soon  coalesce  and  unite  ;  so  that,  after 
beating,  no  trace  of  the  rupture  is  left.  After  having 
been  beaten  for  an  hour  in  a  mold,  until  the  leaves 
have  attained  a  thinness  of  j-nH^o-  part  of  an  inch  in 
thickness,  green  rays  of  light  begin  to  be  transmitted, 
if  the  gold  be  pure  ;  but,  if  largely  alloyed  with  silver, 
rays  of  a  pale-violet  hue  pass  through  the  gold. 

The  membrane  called  "gold-beaters'  skin,"  used 
in  the  make-up  of  the  shoder  and  mold,  is  the  outer 
coat  of  the  caecum  or  blind  gut  of  the  ox.  It  is  im- 
mersed in  a  potash  solution,  and  scraped  with  a  blunt 


l68  DENTAL    METALLURGY. 

knife  to  free  it  from  fat.  It  is  then  stretched  on  a 
frame,  two  membranes  are  glued  together,  treated 
with  camphor  in  isinglass,  and  subsequently  coated 
with  albumen,  and  cut  into  squares  of  five  or  five  and 
a  half  inches,  and  is  ready  for  use.  It  is  stated  that 
the  cseca  of  three  hundred  and  eighty  oxen  are  re- 
quired to  yield  enough  of  the  membrane  to  make  up 
one  mold  of  nine  hundred  and  fifty  pieces,  only  two 
and  one-half  skins  being  obtained  from  each  animal. 
Dryness  is  a  matter  of  great  importance,  and,  as  the 
leaves  are  liable  to  absorb  moisture  from  the  atmos- 
phere, they  require  hot-pressing .  every  time  they  are 
used,  and  if  this  precaution  is  neglected  the  leaf  will 
be  pierced  with  innumerable  holes  or  reduced  to  a 
pulverulent  state. 


CHAPTER   IX. 

SILVER. 
Atomic  Weight,  ioS.     Symbol,  Ag  (Argentum). 

SILVER  may  be  classed  as  next  to  gold  in  its  mani- 
fold uses  and  great  malleability  and  ductility.    It 
has  been  known  from  the  earliest  ages,  and,  al- 
loyed with  certain  proportions  of  copper,  it  has  been 
adopted  by  all  civilized  nations  for  purposes  of  coin- 
age, and  for  articles  of  plate  and  ornamentation. 

Properties  of  Silver. — It  is  distinguished  from  all 
other  metals  by  its  brilliant  whiteness.  Its  specific 
gravity  is  10.53.  in  hardness  it  is  between  gold  and 
copper.  It  is  one  of  the  most  ductile  and  malleable 
of  the  metals, — indeed,  when  calculated  by  weight,  it 
is  not  even  surpassed  by  gold.  For  example,  one 
grain  of  gold  may  be  beaten  out  to  the  extent  of  75 
square  inches,  and  the  same  weight  of  silver  to  98 
square  inches.  Taking  a  cubic  inch  of  gold  at  4900 
grains,  this  gold  leaf  is  8 e flx6 s ft  part  of  an  inch  in 
thickness,  or  about  1200  times  thinner  than  ordinary 
printing-paper.*  But  the  silver,  though  spread  over 
a  larger  surface,  will  be  thicker,  owing  to  the  differ- 
ence of  specific  gravity  between  gold  and  silver.  The 
extent  of  the  malleability  of  gold  and  silver  has  not 
yet  been  definitely  determined,  as  the  means  employed 

*Gold  has,  for  the  sake  of  experiment,  been  beaten  out  to  the  extent 
given  above,  but  the  pi„  of  an  inch,  as  given  on  page  119,  is  as  thin 
as  is  ever  required  for  practical  purposes. 

12  169 


I70  DENTAL    METALLURGY. 

to  test  it  have  failed  before  there  was  any  appearance 
of  the  malleability  of  either  of  them  being  exhausted.* 

In  tenacity  silver  surpasses  gold.  It  fuses  at  about 
18730  F. ,  and  during  the  fusion  absorbs  oxygen  to 
the  extent  of  about  twenty- two  times  its  own  volume  ; 
but  at  the  instant  of  solidification  it  undergoes  consid- 
erable expansion,  while  at  the  same  time  it  parts  with 
the  oxygen,  which  makes  its  escape  through  the  thin 
crust  formed  over  the  fluid  metal,  carrying  with  it 
fine  globules  of  the  metal,  which  may  be  observed  ad- 
hering to  the  sides  of  the  crucible.  It  is  the  best  con- 
ductor of  heat  and  electricity  known.  It  possesses 
no  direct  attraction  for  oxygen  ;  hence  it  is  not  oxid- 
ized by  dry  or  moist  air  at  any  temperature.  It  is,  how- 
ever, oxidized  by  ozone,  and  tarnished  by  air  contain- 
ing sulfuretted  hydrogen,  which  blackens  the  surface 
with  a  superficial  layer  of  sulfid  of  silver,  which  may 
be  removed  by  a  solution  of  cyanid  of  potassium. 

With  the  exception  of  nitric,  silver  is  not  affected 
by  dilute  acids  ;  but  hot  concentrated  sulfuric  acid 
converts  it  into  sulfate  of  silver,  and  when  boiled  with 
strong  hydrochloric  acid  it  dissolves  to  a  slight  extent 
in  the  form  of  chlorid  of  silver,  which  is  precipitated 
by  the  addition  of  water. 

Occurrence  and  Distribution. — In  the  middle  ages 
Austria  was  the  chief  source  from  which  silver  was 
obtained,  as  an  associate  metal  with  lead  At  the 
present  day  the  United  States,  Peru,  and  Mexico 
supply  large  quantities. 

Silver  is  found,  first,  as  native  silver,  occurring  in 
flat  masses  occasionally,  and  sometimes  crystalline  in 
form.      In  this  country  it  occurs  with  native  copper, 

*  W.  Chandler  Roberts,  Assayer  Royal  Mint. 


SILVER.  171 

masses  frequently  being  met  with  in  which  the  two 
metals  are  diffused,  the  silver  showing  in  specks  upon 
the  copper. 

Native  silver  is  usually  free  from  any  considerable 
admixture  with  other  metals,  although  it  invariably 
contains  traces  of  gold,  antimony,  etc.  It  is  also 
found  as  chlorid,  iodid,  and  bromid. 

The  most  common  ores  from  which  silver  is  derived 
are  those  resulting  from  combination  with  sulfur  as 
sulfids.  These  may  be  divided  into  three  kinds  : 
First  may  be  mentioned  the  common  sulfid,  of 
Mexico,  called  vitreous  sulhd.  It  is  a  protosulfid,  is 
very  fusible,  and  readily  yields  silver  when  made  to 
give  up  its  sulfur.  Another  sulfid,  closely  resembling 
the  first,  called  brittle  silver  ore,  is  found  in  South 
America  and  in  some  parts  of  Europe.  It  is  readily 
decomposed  by  heat,  and,  during  exposure  to  high 
temperatures,  evolves  fumes  of  arsenic  and  antimony. 
A  third  sulfid,  found  in  nearly  all  silver  mines  in  the 
form  of  ruby-colored,  transparent  crystals,  is  called 
red  silver  ore,  and  is  associated,  to  some  extent,  with 
oxids  The  composition  of  this  ore  has  been  given  as 
follows  :  Silver,  56  to  62  ;  antimony,  16  to  23  ;  sulfur, 
11  to  14  ;  oxygen,  8  to  10. 

The  chlorid  or  native  horn-silver  is  quite  an 
abundant  ore  of  South  America  (Chili).  It  is  a 
true  chlorid,  and,  like  precipitated  chlorid  of  silver, 
darkens  when  exposed  to  sunlight.  Its  composition 
is  given  as,  silver,  75.3  ;  chlorin,  24.7. 

Methods  of  Separating  Silver  fro?n  its  Ores.  — As 
much  of  the  silver  of  commerce  is  extracted  from  ores 
too  poor  to  admit  of  its  economical  separation  by  any 
process  of  melting  or  fusing,   even  in  regions  where 


172  DENTAL   METALLURGY. 

fuel  is  plenty,  recourse  to  the  method  known  as  "amal- 
gamation" is  necessary.  This  depends  simply  upon 
the  easy  solubility  of  silver  and  associated  metals  in 
mercury.  The  ore  is  crushed  to  powder,  mixed  with 
a  sufficient  quantity  of  common  salt,  and  roasted  at  a 
dull-red  heat  in  a  suitable  furnace.  By  this  treatment 
any  sulfid  of  silver  contained  is  converted  into  chlorid. 
The  mixture,  which  consists  of  much  earthy  matter, 
metallic  oxids,  soluble  salts,  silver  chlorid,  and  metallic 
silver,  is  sifted  and  placed  in  barrels  arranged  to  revolve 
on  axes.  Scraps  of  iron  and  water  are  added,  and  the 
whole  agitated  together  for  the  purpose  of  reducing 
the  silver  chlorid  to  the  metallic  state.  A  sufficient 
quantity  of  mercury  is  then  added,  and  the  agitation 
continued  until  the  metallic  particles  are  dissolved, 
forming  a  fluid  amalgam  which  is  readily  separated 
from  the  mud  or  earthy  matter  by  subsidence  and 
washing.  It  is  then  strained  through  a  strong  linen 
cloth  or  other  suitable  fabric  to  separate  the  fluid  mer- 
cury from  the  more  solid  portions  of  amalgam.  These 
latter  are  subsequently  exposed  to  heat  in  a  retort,  by 
which  the  remaining  mercury  is  distilled  off.  The 
silver,  more  or  less  impure  from  admixture  with  other 
metals  contained  in  the  ore,  is  thus  obtained. 

In  order  to  prevent  loss  during  the  amalgamation 
process,  in  consequence  of  a  tendency  on  the  part  of 
the  mercury  to  combine  with  sulfur,  oxygen,  etc., 
technically  known  as  "  flouring,"  in  which  condition 
it  may  be  washed  away,  together  with  the  silver  it  has 
taken  up,  from  one  to  two  per  cent,  of  sodium  is  added 
to  the  mercury.  The  great  affinity  of  sodium  for  sul- 
fur and  oxygen  prevents  ' '  flouring' '   of  the  mercury. 

Considerable  quantities  of  silver  are  obtained  from 


SILVER.  173 

argentiferous  galena,'''  and,  indeed,  it  may  be  stated 
that  nearly  every  specimen  of  native  lead  sulfid  will 
be  found  to  contain  traces  of  the  nobler  metal.  When 
the  proportion  of  the  precious  metal  present  is  suffi- 
ciently large  to  insure  its  profitable  separation,  the  ore  is 
reduced  as  usual,  t  the  silver  remaining  with  the  lead, 
and  is  then  treated  according  to  a  process  discovered 
by  Mr.  Pattinson,  by  whom  it  was  found  that,  when 
lead  containing  a  considerable  amount  of  silver  is  fused 
and  carefully  stirred  while  it  is  allowed  to  cool  slowly, 
crystals  much  less  rich  in  silver  than  the  mass  before 
melting  will  form,  and  separate  and  subside  to  the 
bottom.  These  crystals  of  poorer  lead  are  removed 
by  means  of  perforated  ladles.  The  silver  is  thus 
concentrated.  This  method  of  separating  silver  from 
lead,  as  practiced  on  a  large  scale,  is  thus  described 
by  Mr.  Makins  in  his  "  Manual  of  Metallurgy  :" 

"A  series  of  iron  pots,  from  nine  to  twelve  in  num- 
ber, are  employed.  These  are  hemispherical,  about 
five  feet  in  diameter,  and  calculated  to  hold  a  charge 
of  about  nine  tons  of  metal  each.  They  are  set  in 
brick  furnaces  adjacent  to  one  another,  but  with  quite 
distinct  flues,  furnaces,  dampers,  etc.  The  lead,  as- 
sorted according  to  its  richness  in  silver,  is  then  placed 
in  the  pots  in  the  following  order  :  Some  lead  contain- 
ing ten  ounces  of  silver  per  ton  having  to  be  worked, 
nine  tons  of  it  would  be  placed  in  the  fifth  pot  and 
melted.  After  complete  fusion  it  is  skimmed  with  a 
perforated  ladle,  which  removes  the  dry  oxids  for  sub- 
sequent reduction,  while  it  permits  the  fluid  lead  to  run 

♦Silver  is  invariably  present  in  this  form  of  lead  ore,  but  not  always  in 
paying  quantities. 
fSee  chapter  on  "  Lead." 


174  DENTAL    METALLURGY. 

back  into  the  pot.  The  fire  is  then  drawn,  and  the 
metal  stirred  while  it  slowly  cools  until  it  begins  to 
thicken.  The  workman  at  this  stage  of  the  operation 
employs  an  iron  ladle  of  eighteen  inches  in  diameter 
by  five  inches  deep,  perforated  with  half-inch  holes, 
and  furnished  with  a  very  long  handle.  This  handle  he 
raises  above  his  head,  sinking  the  bowl  into  the  lead 
until  it  reaches  the  bottom.  Then,  by  using  the 
handle  as  a  lever,  and  depressing  it  as  far  as  possible, 
the  ladle  full  of  crystals  is  brought  into  view,  and,  by 
means  of  a  hook  and  chain  fastened  to  a  crane,  is  sus- 
pended and  left  to  thoroughly  drain,  after  which  the 
crystals  are  turned  into  the  fourth  pot.  This  opera- 
tion is  continued  until  two-thirds  of  the  lead  in  the 
fifth  pot  has  been  passed  over  in  crystals  to  the  fourth 
pot,  under  which  a  fire  is  made  and  the  crystals  again 
melted.  The  remaining  three  tons  of  molten  lead  in 
the  fifth  pot,  which  by  the  separation  of  the  crystals 
contains  silver  equaling  twenty  ounces  per  ton,  is  now 
ladled  into  the  sixth  pot.  The  results  of  the  preceding 
operations  may  be  summed  up  as  follows  :  In  pot  5, 
nine  tons  of  ten-ounce  lead  equals  ninety  ounces  of 
silver,  of  which  six  tons  of  five  ounces  (thirty  ounces 
silver)  works  into  pot  4  ;  and  three  tons  of  twenty 
ounces  (sixty  ounces  silver)  is  ladled  into  pot  6.  The 
work  now  proceeds  until  all  the  pots  are  in  operation. 
Three  tons  of  five-ounce  lead  would  be  added  to  the 
six  tons  passed  into  pot  4,  while  six  tons  of  twenty- 
ounce  lead  would  be  carried  into  pot  6.  Six  tons  from 
pot  6  would  work  into  number  5,  and  three  tons  in 
bottoms  will  be  put  back  into  the  same  pot  from  num- 
ber 4,  filling  it  again  without  the  addition  of  pig-lead. 
The  bottoms  or  portions  which  remain  after  the  ladling 


SILVER.  175 

become  by  that  process  so  rich  in  silver  as  to  often 
contain  six  hundred  ounces  to  the  ton.  This  is  finally 
submitted  to  cupellation,  by  which  means  the  complete 
separation  of  the  silver  is  effected." 

The  cupel  and  its  application  may  be  thus  briefly 
described  :  Bone-ash  is  mixed  with  water,  made  into 
a  cup,  in  a  suitable  mold,  and  dried.  This  is  called 
the  cupel, *  and  has  the  property  of  absorbing  oxids 
when  they  are  combined  with  oxids  of  lead  in  a  state 
of  fusion .  Impure  silver  is  mixed  with  a  certain  quan- 
tity of  lead,  determined  by  the  amount  of  impurity 
supposed  to  exist  in  the  alloy.  The  mixture  is  melted 
in  the  cupel  in  a  current  of  air  until  the  whole  ol  the 
lead  is  converted  into  oxid,  which,  in  a  fused  state, 
sinks  into  the  porous  cupel,  carrying  along  with  it  the 
other  impurities,  the  silver  being  left  behind  in  a  pure 
state.  The  whole  operation  is  based  on  the  absence 
of  attraction  for  oxygen  evinced  by  the  noble  metals 
even  when  exposed  to  high  temperatures,  and  on  the 
affinity  possessed  by  the  base  metals  for  oxygen  under 
similar  conditions. 

Cupellation  may  be  accomplished  either  in  a  muffle 
arranged  with  reference  to  the  passage  of  a  current  of 
air,  so  that  oxygen  may  be  freely  supplied  to  the 
melted  metal,  or  it  maybe  performed  under  the  oxid- 
izing flame  of  the  blow-pipe.  The  latter  operation 
is  often  employed  in  blow-pipe  analysis.  A  certain 
amount  of  the  alloy  is  mixed  with  about  four  times  its 
weight  of  pure  lead,  and  then  placed  on  the  cupel  and 
the  oxidizing  flame  of  the  blow-pipe  directed  on  it. 
The  oxidizing  process  soon  begins,  and  in  about  thirty 
minutes  all  the  lead  will  be  converted  into  litharge, 

*  These  may  be  obtained  at  the  chemists'  furnishing-shops  ready  for  use. 


176  DENTAL    METALLURGY. 

which  is  fusible,  and  is  readily  absorbed  into  the 
porous  substance  of  the  cupel,  carrying  with  it  all  the 
oxidizable  metals  that  may  be  present.  At  this  point, 
the  button  having  parted  with  every  trace  of  the 
latter,  assumes  an  exceedingly  bright  appearance, 
technically  called  the  "brightening  of  the  button," 
thus  offering  a  certain  means  of  ascertaining  when 
the  process  of  cupellation  is  complete.  Cupellation, 
under  the  oxidizing  flame  of  the  blow-pipe,  for  quan- 
titative discrimination,  requires  careful  management, 
particularly  when  the  silver  has  parted  with  the  base 
metals  and  approaches  a  state  of  purity.  For  it  is  at 
this  stage  of  the  operation  that  the  well-known  prop- 
erty of  melted  silver,  of  absorbing  oxygen  from  the 
atmosphere,  and  then  parting  with  it  as  it  approaches 
the  point  of  solidification,  may  be  observed.  The 
giving-off  of  the  absorbed  oxygen  is  what  causes 
"  sputtering,"  by  which  minute  globules  of  the  metal 
are  thrown  off  and  lost,  thus  rendering  the  assay 
inaccurate. 

Besides  the  method  of  obtaining  silver  above  de- 
scribed, the  metal  may  be  obtained  by  converting 
sulfids  into  chlorid,  the  latter  being  easily  reduced  to 
metallic  silver  by  the  wet  method.  The  sulfid  is  also 
sometimes  converted  into  sulfate,  when  the  silver  may 
be  reduced  from  the  solution  by  precipitation. 

Another  method  of  separating  silver  from  its  ores 
consists  in  roasting  the  latter  with  common  salt  to 
convert  the  silver  into  chlorid,  which  is  dissolved  out 
of  the  mass  by  means  of  a  strong  solution  of  chlorid 
of  sodium  ;  the  silver  is  then  recovered  in  the  metallic 
state  by  precipitating  with  copper.  Hyposulfite  of 
soda  has  also  been  employed  to  dissolve  out  the  chlo- 


SILVER.  177 

rid  of  silver,  the  resulting  solution  being  precipitated 
by  sulfid  of  sodium,  yielding  sulfid  of  silver,  which 
requires  roasting  to  drive  off  the  sulfur  and  liberate 
the  metallic  silver. 

Compounds  of  Silver. — There  are  three  compounds 
of  silver  with  oxygen  :  the  suboxid,  AgO  ;  the  oxid, 
Ag,0  ;  and  the  peroxid,  which  is  thought  to  have  the 
formula  of  Ag,0,.  The  oxid  is  the  only  one  having 
any  practical  importance.  Being  the  base  contained 
in  the  salts  of  silver,  it  is  obtained  by  adding  caustic 
potassa  or  baryta-water  to  a  solution  of  nitrate  of 
silver. 

Silver  nitrate  (AgN03)  is  prepared  by  dissolving 
silver  in  nitric  acid  by  the  aid  of  gentle  heat,  after 
which  it  is  evaporated  to  dryness  or  until  it  crystal- 
lizes. These  crystals  are  colorless,  transparent,  and 
soluble  in  an  equal  weight  of  cold  and  in  half  the 
quantity  of  boiling  water.  They  are  also  soluble  in 
alcohol.  Nitrate  of  silver  is  fusible,  and  when  poured 
into  cylindrical  molds  forms  the  lunar  caustic  employed 
by  surgeons.  At  high  temperatures  (red  heat)  it  is 
decomposed,  yielding  pure  metallic  silver. 

Silver  sulfate  (Ag,S04)  is  prepared  by  boiling  metal- 
lic silver  in  sulfuric  acid. 

Silver  sulfid  is  remarkable  for  being  so  soft  and 
malleable  that  medals  may  be  struck  from  it.  It  may 
be  formed  as  a  black  precipitate  by  the  action  of  hy- 
drogen sulfid  (H.,S)  upon  a  solution  of  silver  nitrate, 
or  it  may  be  formed  by  heating  silver  with  sulfur  in  a 
covered  crucible.  It  is  the  affinity  existing  between 
these  two  elements  which  renders  the  combination  of 
silver  and  vulcanizable  rubbers  impracticable.  Silver 
sulfid  is  not  soluble  in  dilute  sulfuric  or  hydrochloric 


178  DENTAL   METALLURGY. 

acid,  but  is  readily  dissolved  by  nitric  acid.     Metallic 
silver  also  dissolves  sulfid  of  silver  when  melted  with  it. 

Silver  chlorid  (AgCl)  is  the  form  into  which  silver 
is  commonly  converted  in  separating  it  from  other 
metals  or  from  its  ores.  It  is  a  white,  curdy  precipi- 
tate, and  may  be  obtained  from  a  solution  of  the  ni- 
trate by  the  addition  of  sodium  chlorid  or  hydrochloric 
acid.  When  freshly  prepared  it  is  perfectly  white, 
but  soon  darkens,  and  eventually  becomes  quite  black 
by  exposure  to  solar  light,  parting  with  a  portion  of 
its  chlorin,  and  becoming  a  subchlorid  (Ag2Cl). 

Silver  chlorid  may  also  be  formed  by  suspending  a 
silver  leaf  in  a  glass  vessel  containing  chlorin  gas,  and 
when  thus  prepared  it  is  not  blackened  by  exposure 
to  light.  Argentic  chlorid  is  fusible  at  5000  F.  A 
much  higher  heat  converts  it  into  vapor,  but  does  not 
decompose  it.     It  is  soluble  in  ammonia. 

Discrimination. — The  chlorids  and  hydrochloric 
acid  precipitate  white  argentic  chlorid,  and  so  delicate 
is  the  test  that  when  one  part  of  silver  is  dissolved  in 
200,000  times  its  weight  of  water  it  may  be  readily 
detected  by  the  opalescence  which  is  imparted  to  the 
fluid  by  the  precipitant.  This  precipitate  is  always 
changed  to  a  violet-black  by  exposure  to  light,  but 
the  presence  of  mercury*  will  prevent  discoloration. 
It  is  insoluble  in  nitric  acid,  but  is  readily  soluble  in 
ammonia,  and  may  be  fused  to  a  horny  mass  without 
decomposition. 

Sulfuretted  hydrogen  added  to  a  solution  contain- 
ing silver  throws  down  a  black  precipitate  of  silver 
sulfid,  which  is  not  soluble  in  dilute  acids,  alkalies,  or 
potassic  cyanid.      Sulfuric   acid   at  a  temperature  of 

*  Mercurous  chlorid. 


SILVER.  179 

2120  F.  will,  however,  dissolve  it,  with  separation  of 
the  sulfur. 

Ammonia  or  potassa,  when  employed  as  a  precipi- 
tant, throws  down  a  brown  oxid,  insoluble  in  the 
latter,  but  soluble  in  the  former,  and  if  freely  exposed 
to  air  this  solution  will  deposit  fulminating  silver. 

The  blow-pipe  is  frequently  used  in  the  discrimina- 
tion of  silver  compounds,  which  when  heated  on  char- 
coal with  sodic  carbonate  yield  a  bright  bead  of 
metallic  silver,  often  accompanied  by  a  red-colored 
deposit  on  the  charcoal. 

Quantitatively,  the  estimation  of  silver  may  be  ac- 
complished either  by  the  usual  humid  process  or  by 
assaying.  The  first  consists  in  precipitating  the  metal 
as  chlorid,  which  is  to  be  separated  and  weighed.  The 
precipitation  is  effected  as  follows  :  The  silver  solution 
is  acidulated  by  nitric  acid  ;  hydrochloric  acid  or  sodic 
chlorid,  slightly  in  excess,  is  then  added  ;  but,  as  silver 
chlorid  is  to  a  certain  extent  soluble  in  either  of  these, 
an  undue  excess  must  be  avoided  The  chlorid  must 
now  be  carefully  and  repeatedly  washed  and  filtered 
in  a  thoroughly  dry  filter,  previously  weighed.  After 
the  solution  has  passed  through  the  filter,  the  latter 
with  its  contents  is  dried  and  weighed,  and  the  weight, 
minus  the  weight  of  the  filter,  will  be  the  quantity  of 
silver  chlorid  present. 

The  dry  method  or  assaying  process  consists  in 
forming  an  alloy  of  the  silver  with  lead,  and  is  espe- 
cially applicable  to  ores  and  the  sweep  of  the  dentist's 
laboratory.  The  specimen  to  be  treated  is  heated  with 
from  twelve  to  thirty  times  its  weight  of  pure  granu- 
lated lead  in  a  bone-ash  cupel,  which  is  placed  in  a 
muffle  so  arranged  that  a  current  of  atmospheric  air 


l8o  DENTAL   METALLURGY. 

may  pass  freely  over  the  vessel  and  oxidize  the  lead. 
This  oxid  of  lead,  being  quite  fusible,  combines  with 
any  base  metal  present  and  oxidizes  it,  uniting  subse- 
quently with  the  oxid  as  a  fusible  slag,  while  the  gold 
or  silver  will  be  held  by  the  unoxidized  portion  of  the 
lead.  In  the  treatment  of  specimens  of  alloys,  such 
as  plate  or  coin,  a  quantity  of  the  specimen  is  accu- 
rately weighed  and  mixed  with  from  four  to  five  times  its 
weight  of  pure  granulated  lead.  It  is  then  placed  in  the 
cupel  and  exposed  to  heat,  as  above  described,  until 
all  the  lead  is  oxidized  or  converted  into  litharge,  when 
the  remaining  button  assumes  the  brilliant  appearance 
of  surface  before  alluded  to,  which  denotes  that  the 
base  metals  or  oxidizable  constituents  have  been  oxid- 
ized and  taken  up  by  the  lead  oxid.  This  button  is 
then  to  be  weighed  by  means  of  a  delicate  assay 
balance,  and  the  loss  of  weight  shows  the  proportion 
of  alloy  that  was  present. 

Pure  Silver. — Pure  silver,  which  is  reckoned  as  iooo 
fine,  may  be  obtained  from  standard  or  other  grades 
of  silver  by  dissolving  them  in  nitric  acid  slightly 
diluted  with  water,  the  solution  being  much  facilitated 
by  exposure  to  gentle  heat.  If  gold  be  associated 
with  the  alloy  it  will  be  found  at  the  bottom  of  the 
vessel,  in  which  case  it  will  be  necessary  to  use  a 
syphon  to  remove  the  argentic  nitrate  solution.  The 
silver  is  now  to  be  precipitated  in  the  form  of  chlorid 
by  the  addition  of  an  excess  of  common  salt.  When 
all  has  subsided  the  liquid  is  carefully  poured  off,  and 
the  chlorid  thoroughly  washed  to  remove  all  traces  of 
acid.  The  chlorid  is  then  placed  in  water  acidulated 
with  hydrochloric  acid  (an  ounce  of  chlorid  requiring 
six  to  eight  ounces  of  water)  and  pieces    of  clean 


SILVER.  l8l 

wrought  iron  put  in  it,  when  a  copious  evolution  of 
hydrogen  follows,  which,  uniting  with  the  chlorin  of 
the  argentic  chlorid,  liberates  metallic  silver.  The 
latter  should  not  be  disturbed  until  the  last  particle  of 
it  is  thus  reduced,  when  it  will  be  found  to  be  a  spongy- 
mass.  The  undissolved  iron  should  now  be  carefully 
removed,  the  ferrous  and  ferric  chlorid  carefully  de- 
canted, and  the  silver  washed  in  hot  water  containing 
about  one-tenth  its  bulk  of  hydrochloric  acid.  This 
is  repeated  several  times,  and  finally  the  silver  is  again 
thoroughly  washed  with  pure  hot  water.  The  silver, 
after  drying,  is  then  ready  for  melting,  and  if  care 
has  been  observed  in  the  process  it  will  be  found  to  be 
of  a  fineness  of  999.7  parts  in  1000,  the  0.3  of  impurity 
present  being  due  to  traces  of  iron.  The  chlorids  may 
be  acidulated  with  sulfuric  acid,  and  reduced  with  zinc 
instead  of  iron. 

Another  method  of  precipitating  silver  in  the  metallic 
form  consists  in  placing  a  sheet  of  copper  in  a  solution 
of  argentic  nitrate.  The  metal  is  thrown  down  in  a 
crystalline  form.  Silver  thus  obtained  is  never  free 
from  traces  of  copper. 

Pure  silver  can  only  be  obtained  from  samples  of  a 
lower  grade  by  fusing  the  pure  chlorid  with  sodic 
carbonate.     The  reaction  is  shown  in  the  equation  : 
2AgCl+Na,C03=Ag2+2NaCl+0+C02. 

Owing  to  the  copious  evolution  of  carbonic  acid  gas 
which  takes  place  during  the  decomposition,  some  of 
the  silver  may  be  thrown  from  the  crucible,  and  loss 
may  occur  by  the  absorption  by  the  crucible  of  some 
of  the  fused  chlorid.  To  avoid  this  the  sides  of  the 
vessel  should  be  coated  with  a  hot  saturated  solution 
of  borax. 


152  DENTAL    METALLURGY. 

A  composition  of  ioo  parts  of  argentic  chlorid,  70.4 
of  calcic  carbonate  (chalk),  and  4.2  of  charcoal,  has 
been  recommended  as  a  means  of  obtaining  pure  sil- 
ver. This  mixture  is  heated  to  dull  redness  for  thirty 
minutes,  and  then  raised  to  full  redness  ;  carbonic 
acid  and  carbonic  oxid  are  given  off ;  the  calcic  chlorid 
is  converted  into  calcic  oxychlorid,  underneath  which, 
in  the  bottom  of  the  crucible,  will  be  found  the  button 
of  pure  silver. 

Alloys  of  Silver. — In  consequence  of  its  softness, 
silver  in  the  pure  state  is  liable  to  considerable  loss  by 
attrition.  For  all  useful  purposes,  however,  the  requi- 
site amount  of  hardness  may  be  conferred  upon  it  by 
the  addition  of  a  small  proportion  of  copper.  Thus, 
silver  for  coinage  and  manufacturing  purposes  usually 
contains  in  1000  parts  from  900  to  925  of  silver  and 
from  75  to  100  of  copper.  The  term  "standard  sil- 
ver" refers  to  the  metal  thus  alloyed  with  copper, 
that  of  the  United  States  coinage  being  silver  900 
parts,  copper  100. 

Previous  to  the  introduction  of  vulcanized  rubber  as 
a  base  for  artificial  dentures,  standard  silver  was  much 
employed  in  the  United  States  for  temporary  dentures, 
when  cheapness  was  an  important  consideration.  In 
England  a  much  more  durable  alloy  is  used,  in  which 
the  alloying  metal  is  platinum,  in  the  proportion  of 
from  three  to  ten  grains  of  the  latter  to  each  penny- 
weight of  silver.  The  advantages  possessed  by  this 
alloy  over  ordinary  standard  silver  may  be  summed 
up  as  follows  :  It  resists  wear  better,  and  not  even  a 
suspicion  can  be  reasonably  entertained  of  any  ill 
effects  occurring,  either  locally  or  to  the  general  sys- 
tem, from  its  presence  in  the  mouth.     It  permits  of 


SILVER.  183 

the  employment  of  a  higher  grade  of  solder,  and  it  is 
a  much  more  rigid  alloy  than  ordinary  standard  or 
coin  silver.  Hence  it  makes  a  stronger  artificial  den- 
ture, which  is  less  likely  to  have  its  adaptation  im- 
paired by  bending.  But,  while  silver  is  improved  in 
some  respects  when  platinum  is  the  sole  alloying  com- 
ponent, it  must  not  be  supposed  that  its  affinity  for 
sulfur  is  thus  materially  lessened,  or  that  its  tendency 
to  blacken  when  brought  into  contact  with  that  ele- 
ment or  its  compounds  is  obviated.  Indeed,  it  may  be 
stated  that  platinum  added  to  silver  in  such  small 
quantities  does  not  wholly  protect  the  latter  from  the 
action  of  its  ordinary  solvents.  Such  an  alloy  of  sil- 
ver, for  instance,  would  not  only  be  readily  dissolved 
by  nitric  acid,  but  the  platinum  also,  though  unaffected 
ordinarily  by  that  menstruum,  would  readily  yield  to 
it  when  combined  with  silver. 

This  alloy  of  silver,  which  is  known  in  England  as 
"dental  alloy,"  often  contains  from  twenty-five  to 
thirty  per  cent,  of  platinum.  To  separate  the  latter 
metal  from  the  silver  the  alloy  is  dissolved  in  nitric 
acid,  which  on  the  addition  of  heat  will  dissolve  all 
of  the  silver  with  about  ten  per  cent,  of  the  platinum. 
On  introducing  a  bar  of  copper  into  this  solution, 
the  silver  and  platinum  are  quickly  precipitated  in  a 
metallic  state.  This  precipitate  is  again  placed  in 
nitric  acid,  which  redissolves  the  silver,  leaving  the 
platinum  untouched,  which,  however,  may  be  dissolved 
with  the  other  fifteen  or  twenty  per  cent,  of  the  plat- 
inum left  at  first,  in  aqua  regia.  Precipitating  by  an 
excess  of  ammonium  chlorid,  evaporating  to  dry- 
ness, and  igniting,  yields  pure  platinum.  The  silver 
may  be  recovered  in  the  usual  way  by  precipitation  in 


184  DENTAL    METALLURGY. 

the  form  of  a  chlorid,  which  may  be  easily  reduced  to  a 
metallic  state  by  treating  with  a  plate  of  zinc  in  acidu- 
lated water. 

It  is  a  somewhat  common  belief  that  the  putting 
together  of  silver  and  platinum  in  the  formation  of  an 
alloy  of  this  kind,  owing  to  the  infusibility  of  platinum 
and  the  wide  difference  in  the  fusing-points  of  the  two, 
is  a  matter  of  great  difficulty.  It  should  be  borne  in 
mind,  however,  that  between  the  metals  more  or  less 
affinity  exists,  especially  at  high  temperatures  ;  hence 
it  is  only  necessary  to  introduce  the  platinum,  rolled 
into  thin  ribbons,  into  the  crucible  containing  the  sil- 
ver in  a  state  of  complete  fusion,  and  the  platinum 
will  be  observed  to  quickly  fuse  and  mix  with  the 
other  metal.  It  is  sometimes  thought  advisable  to  add 
larger  proportions  of  platinum  than  the  quantity  here 
given.  This  may  be  done  by  adding  the  platinum 
until  the  alloy  becomes  infusible,  and  this  result  will 
be  attained  as  soon  as  sufficient  platinum  is  added  to 
raise  the  fusing-point  of  the  alloy  above  the  capacity 
of  the  ordinary  melting  apparatus. 

Von  Eckart's  alloy,  employed  to  some  extent  in 
France  as  a  base  for  artificial  dentures,  is  composed 
of  the  following  proportions  :  silver,  3.53  ;  platinum, 
2.40;  and  copper,  11. 71.  It  is  very  elastic  (which 
property  it  does  not  lose  by  annealing),  and  can  be 
highly  polished. 

Silver  Solders. — When  the  plate  to  be  united  con- 
sists of  pure  silver  alloyed  with  platinum,  the  solder 
may  be  formed  of  the  standard  metal  (coin),  with  the 
addition  of  from  one-tenth  to  one-sixth  its  weight  of 
zinc,  according  to  the  proportion  of  platinum  contained 
in  the  alloy.     Silver  solders  are,   however,  generally 


66  parts. 

30      " 

10      " 

6  dwts. 

2      " 

1  dwt. 

5^  dwts. 

40  grs. 

SILVER.  185 

composed  of  silver,  copper,  and  zinc,  or  silver  and 
brass,  in  variable  proportions,  of  which  the  following 
are  examples  : 

No.  1.* 

Silver 

Copper         

Zinc 

No.  2.f 

Silver 

Copper        

Brass 

No.  3. 

Silver 

Brass  wire 

In  putting  together  the  constituents  of  silver  solders, 
the  affinity  for  oxygen  manifested  by  zinc,  brass,  and 
copper,  when  exposed  to  high  temperatures,  should 
be  remembered,  and  in  order  to  guard  against  loss  the 
mode  of  procedure  should  be  as  follows  :  The  silver, 
placed  in  a  clean  crucible,  with  a  sufficient  quantity  of 
borax  to  cover  it,  should  be  thoroughly  fused,  and, 
without  permitting  it  to  cool  in  the  least,  the  zinc, 
brass,  or  copper,  as  the  case  may  be,  should  be  quickly 
added.  Before  pouring,  it  should  be  shaken  or  agitated 
to  insure  admixture.  When  cool,  it  may  be  removed 
from  the  ingot-mold  and  rolled  into  plate  of,  say  No. 
27  of  the  standard  gauge. 

The  surface  of  standard  silver  may  be  whitened  by 
being  heated  and  immersed  in  dilute  sulfuric  acid.  It 
is  in  this  way  that  frosted  silver  is  produced.  The 
acid,  dissolving  the  oxid  of  silver  from  the  surface, 
leaves  a  quite  pure  superficial  film. 

*  Richardson's  Treatise  on  Mechanical  Dentistry. 
t Ibid. 

13 


186  DENTAL   METALLURGY. 

Silver  may  be  deposited  upon  the  surface  of  another 
metal  by  connecting  the  article  to  be  silvered  with  the 
negative  (zinc)  pole  of  the  galvanic  battery,  and  then 
immersing  it  in  a  solution  made  by  dissolving  cyanid 
of  silver  in  a  solution  of  cyanid  of  potassium.  The 
current  decomposes  the  argentic  cyanid,  and  the 
metal  is  deposited  upon  the  object  connected  with  the 
negative  pole.  During  this  decomposition  the  cyano- 
gen liberated  at  the  positive  (copper  or  platinum)  pole 
acts  upon  a  silver  plate  with  which  this  pole  is  con- 
nected, the  quantity  of  silver  dissolved  at  this  pole 
being  precisely  equal  to  that  deposited  at  the  opposite 
pole  ;  the  silvering  solution  is  always  maintained  at  the 
same  strength. 


CHAPTER  X. 

PLATINUM. 
Atomic  Weight,  197.6.    Symbol,  Pt. 

PLATINUM  is  found  in  nature  in  flattened  grains 
of  varying  sizes,  more  or  less  alloyed  with  pal- 
ladium, rhodium,  ruthenium,  davyum,  and  irid- 
ium.* It  occurs  in  Brazil,  Peru,  Australia,  and  Cal- 
ifornia. Russia,  however,  furnishes  the  largest  supply 
of  platinum,  from  the  Ural  Mountains.  It  was  dis- 
covered in  1736  by  Anton  Ulloa,  at  Choco,  in  South 
America  ;  but,  in  consequence  of  its  infusibility  and 
unworkable  nature,  no  use  was  made  of  it,  and  its 
presence  in  mining  products  was  considered  a  hind- 
rance. Dr.  Wollaston  devised  the  first  practical  pro- 
cess of  working  it,  and  in  1859  Deville  and  Debray 
published  improved  methods  of  fusing  large  quantities 
of  platinum. 

Wollaston' s  method,  which  consists  of  a  series  of 
chemical  and  mechanical  processes  of  a  rather  com- 
plicated nature,  may  be  thus  described  :  The  ore  is 
first  heated  with  nitric  acid  to  dissolve  any  copper, 
lead,  iron,  or  silver.  It  is  then  washed  and  heated 
with  hydrochloric  acid  to  remove  any  magnetic  iron  ore 
that  may  be  present ;  after  which  the  ore  is  to  be  treated 
with  nitro-hydrochloric  acid  diluted  with  an  equal 
bulk  of  water  to  prevent  the  iridium,  which  is  gen- 

*  A  group  of  rare  metals  only  found  in  platinum  ores,  and  known  as  the 
"  platinum  metals." 

187 


1 88  DENTAL   METALLURGY. 

erally  present,  from  being  dissolved.  The  propor- 
tions of  acids  are  one  hundred  and  fifty  parts  of 
hydrochloric  to  forty  parts  of  nitric.  Three  or  four 
days'  digestion,  aided  by  gentle  heat,  is  necessary  to 
complete  solution.  The  suspended  matter,  generally 
consisting  of  iridium,  is  allowed  to  subside,  when  the 
solution  may  be  syphoned  off. 

Ammonic  chlorid*  is  next  added  as  a  precipitant, 
and  throws  down  the  yellow  crystalline  ammonio- 
platinic  chlorid,  which  is  readily  decomposable  by 
heat,  yielding  platinum  in  a  finely-divided  state. 

The  liquid  from  which  the  precipitate  is  obtained 
will  still  be  found  to  contain  about  eleven  parts  of 
platinum,    together   with    all   the   associated   metals. 
These  are  all  thrown   down  by  means  of  a  plate  of 
zinc,    and  washed    carefully  and   again    dissolved  in 
nitro-hydrochloric  acid.     A  small  quantity  of  strong 
hydrochloric  acid  is  added  to  avoid  precipitation  of 
lead  or  palladium,  when  precipitation  of  the  remaining 
platinum  may  be  again  effected  by  ammonic  chlorid. 
This  precipitate  will  require  careful  washing  in  cold 
water  to   remove  iridium,  which  during  the  process 
forms  a  double  salt  with  the  ammonic  chlorid. 

The  next  stage  in  the  operation  consists  in  separat- 
ing the  metal  from  the  ammonia  salt  by  ignition,  and, 
as  it  is  important  to  the  success  of  the  subsequent 
working  that  the  precipitate  shall  remain  in  a  finely- 
divided  state,  too  high  a  degree  of  heat  must  be 
avoided,  as  otherwise  cohesion  of  the  particles  will  take 
place.  Ignition  is  generally  accomplished  by  the  fol- 
lowing means  :  The  precipitate  is  heated  in  a  graphite 
crucible  until  nothing  remains  but  the  finely-divided 

*  About  forty  parts. 


PLATINUM.  189 

platinum.  This  is  powdered,  should  it  be  found  some- 
what lumpy,  in  a  wooden  mortar  with  a  wooden  pestle, 
sifted  through  a  fine  lawn  sieve,  and  mixed  with  water 
to  the  consistence  of  a  stiff  paste.  This  is  placed  in  a 
brass  mold  with  a  slightly  tapering  cylindrical  cavity 
about  seven  inches  in  length,  provided  with  a  loosely- 
fitting  steel  stopper,  which  enters  to  the  depth  of  a 
quarter  of  an  inch.  The  mold  is  first  oiled  and  set  up 
in  a  vessel  of  water.  The  platinum  mud  is  then  intro- 
duced, and  as  it  settles  into  the  water  air  is  displaced, 
and  the  platinum  is  thus  made  to  fill  every  part  of  the 
mold.  The  water  is  allowed  to  drain,  and  its  removal 
may  be  aided  by  pressure.  Ultimately,  however,  the 
mold  is  placed  in  a  press  worked  by  a  powerful  lever, 
by  which  the  mass  sustains  an  enormous  pressure, 
after  which  the  plug  and  the  column  of  platinum  are 
removed  by  gently  tapping  the  mold.  It  is  then 
heated  in  a  charcoal  fire,  in  order  to  thoroughly  dry 
it  and  to  burn  off  any  adherent  oil. 

The  next  step,  which  depends  upon  the  quality  of 
welding  possessed  by  platinum,  consists  in  heating  the 
porous  cylinder  in  a  blast-furnace  to  white  heat,  when 
it  is  removed,  set  upright  on  an  anvil,  and  hammered 
on  the  ends  in  order  to  weld  the  particles  ;  after  which 
it  is  coated  with  a  mixture  of  borax  and  carbonate  of 
potash,  and  again  heated  for  the  purpose  of  removing 
traces  of  iron,  which  is  dissolved  by  the  mixture,  the 
latter  being  removed  by  immersion  in  dilute  sulfuric 
acid.  The  bar  of  platinum  is  now  ready  for  use,  and 
may  be  rolled  or  hammered. 

It  may  readily  be  surmised  that  so  imperfect  a  means 
of  obtaining  a  solid  bar  of  metal  as  the  latter  part  of 
the  operation  just  described  cannot  always  be  relied 


I90  DENTAL     METALLURGY. 

upon  for  the  production  of  a  uniform  and  solid  speci- 
men ;  and,  indeed,  platinum  prepared  in  this  way, 
though  of  great  purity,  is  liable  to  blister  upon  its 
surface,  this  being  probably  due  to  minute  globules  of 
air  incased  in  the  body  of  the  ingot  during  the  forging, 
which,  during  the  conversion  of  the  ingot  into  plate 
by  means  of  rollers,  are  elongated  and  spread  out  in 
the  form  of  blisters. 

The  dry  metallurgic  operations  of  Deville  and  De- 
bray  consist  in  heating  in  a  reverberatory  furnace  about 
two-hundred-weight  of  platinum  ore  with  an  equal 
weight  of  galena  (sulfid  of  lead).  When  the  ore  is 
sufficiently  heated  (to  bright  redness),  portions  of  the 
galena  are  added  and  mixed  with  the  ore  by  constant 
stirring.  An  equal  quantity  of  litharge  is  next  added, 
in  order  to  supply  oxygen  to  the  sulfur  of  the  lead  ore, 
which  passes  off  as  sulfurous  anhydrid,  reducing  all 
of  the  lead  which  combines  with  the  platinum.  After 
remaining  in  a  state  of  fusion  for  a  short  time  the  upper 
portion  is  ladled  off,  and  will  be  found  to  consist  of  an 
alloy  of  lead,  platinum,  and  smaller  portions  of  palla- 
dium and  silver,  the  latter  being  introduced  from  the 
galena,  which  always  contains  more  or  less  silver. 
The  heavier  metals  of  the  platinum  group,  by  their 
greater  density,  subside  to  the  bottom. 

Cupellation  is  now  resorted  to  in  order  to  separate 
the  platinum  from  the  lead.  This  consists  of  two 
distinct  operations.  The  first  is  performed  at  the 
ordinary  furnace-temperature,  and  is  continued  until 
by  loss  of  lead  the  fusing-point  of  the  remaining  alloy 
rises  to  such  an  extent  that  a  state  of  fusion  can  no 
longer  be  maintained.  The  second  and  final  opera- 
tion is  performed  in   an  apparatus  which  serves  the 


PLATINUM.  191 

purpose  of  both  furnace  and  cupel.  It  is  formed  of 
blocks  of  thoroughly  burned  lime.  In  form  it  may  be 
described  as  a  sort  of  basin  or  concavity  with  a  similar 
piece  for  a  cover.  The  lower  part  is  intended  for  the 
reception  of  the  metal  ;  through  the  center  of  the 
upper  portion  or  cover  pass  the  tubes  for  the  oxyhy- 
drogen  jet,  while  the  lower  portion  is  provided  with  a 
lip  or  spout  for  pouring  the  melted  metal.  The  tubes 
which  pass  through  the  top  for  the  transmission  of  the 
two  gases  are  generally  formed  of  copper,  with  plat- 
inum tips.  The  outer  and  lower  tube  carries  hydro- 
gen, while  the  inner  and  upper  one  carries  a  jet  of 
oxygen,  into  the  middle  of  the  flame.  The  tubes  are 
furnished  with  stop-cocks,  so  that  the  supply  may  be 
regulated.  When  the  object  is  merely  to  fuse  some 
scraps  of  platinum,  the  lime-furnace  is  first  put  to- 
gether, the  hydrogen  jet  is  lighted,  oxygen  is  then 
turned  on,  and  the  interior  of  the  apparatus  soon 
becomes  heated.  The  platinum  is  then  introduced 
in  pieces  through  a  small  hole  at  the  side,  and  quickly 
fuses  after  entering  the  furnace. 

When  used  as  a  cupel  the  lime  absorbs  the  impuri- 
ties, and  the  platinum  is  kept  in  a  state  of  fusion  until 
all  the  lead  is  oxidized,  when  the  metal  may  be  poured 
from  the  lime-cupel  into  an  ingot- mold  formed  of  coke 
or  plates  of  lime.  Some  difficulty  may  be  experienced 
at  the  moment  of  pouring,  in  consequence  of  the 
dazzling  white  surface  of  the  molten  metal.  From 
seven  to  eight  pounds  may  be  melted  in  this  way  in 
from  forty  to  sixty  minutes. 

Although  such  metals  as  palladium,  osmium,  gold, 
silver,  and  lead  are  volatilized  at  the  intense  heat  used, 
it  has  been  found  that  platinum  obtained  by  the  Deville- 


192 


DENTAL   METALLURGY. 


Debray  method  is  not  as  pure  as  that  obtained  by 
Wollaston's  plan. 

Properties. — Platinum  is  somewhat  whiter  than  iron. 
It  is  exceedingly  infusible,  requiring  the  flame  of  the 


Fig.  19. 


■=5*=^ 


compound  blow-pipe  (oxyhydrogen)  to  render  it  fluid. 
In  both  the  hot  and  cold  states  it  is  exceedingly  mal- 
leable and  ductile.*  It  is  the  heaviest  substance  in 
nature,  its  specific  gravity  being  21.5,  and  it  is  ex- 

*  Wollaston,  in  endeavoring  to  substitute  platinum  for  the  spider's  web 
usually  employed  in  micrometers,  made  platinum  wire  finer  than  had 
hitherto  been  obtained.  This  was  accomplished  by  forming  a  coating  of 
silver  upon  a  platinum  wire,  and  then  passing  it  through  the  draw-plate, 
after  which  he  dissolved  the  silver,  leaving  the  platinum  the  I  of  an 
inch  in  diameter,  a  mile  of  which,  notwithstanding  the  high  specific 
gravity  of  the  metal,  would  only  weigh  a  single  grain. 


PLATINUM.  193 

ceeded  in  tenacity  only  by  iron  and  copper.  No 
single  acid  attacks  it,  and  it  is  unaffected  by  air  or 
moisture  at  any  temperature.  It  is  therefore  of 
great  value  in  the  construction  of  chemical  vessels. 

At  bright-red  heat  platinum  welds  quite  readily,  and 
injured  vessels  may  be  repaired  in  this  way.  In  the 
finely-divided  state,  as  obtained  by  Wollaston's  pro- 
cess, it  may  be  made  into  small  vessels  by  pressing  the 
pulverulent  metal  into  suitable  molds,  heating  and 
hammering  to  complete  the  welding  of  the  particles. 

Platinum  possesses  the  remarkable  property  of  in- 
ducing chemical  combination  between  oxygen  and 
other  gases.  Even  in  the  compact  condition  it  pos- 
sesses this  quality,  as  demonstrated  by  the  familiar 
experiment  of  suspending  a  coil  of  platinum  wire  in 
the  flame  of  a  spirit-lamp,  and  suddenly  extinguishing 
the  flame  as  soon  as  the  metal  becomes  entirely  heated, 
wnen,  by  inducing  the  combination  of  the  vapor  of 
the  spirit  with  oxygen,  the  wire  will  continue  to  glow. 
An  instantaneous  light  apparatus  has  been  made  in 
which  a  jet  of  hydrogen  is  thrown  upon  a  ball  of 
spongy  platinum  ;  the  latter  induces  combination  be- 
tween the  oxygen  condensed  between  its  pores  and  the 
spirit-vapor,  and  ignition  takes  place. 

Platinum-black,  in  which  the  metal  exists  in  an  ex- 
ceedingly fine  state  of  division,  possesses  this  power 
of  promoting  combination  of  oxygen  with  other  gases 
to  the  highest  degree.  In  this  form  it  is  capable  of 
absorbing  eight  Hundred  times  its  volume  of  oxygen. 
No  combination,  however,  takes  place  between  the 
two,  the  gas  being  merely  condensed  within  the  pores 
of  the  metal  ready  for  combination  with  other  bodies  ; 
hence,  if  a  jet  of  hydrogen  be  thrown  upon  a  small 


194  DENTAL   METALLURGY. 

lump  of  this  powder,  ignition  ensues.  During  the 
operation  of  melting,  platinum  absorbs  oxygen  and 
gives  it  off  in  cooling,  ''sputtering,"  as  silver  does 
under  like  conditions. 

The  proper  solvent  for  platinum  is  nitro-hydrochloric 
acid,  the  chlorin  evolved  being  the  active  agent. 
Alloyed  with  silver,  however,  platinum  will  be  dis- 
solved in  nitric  acid,  and  when  platinum  is  found  in 
gold  as  an  alloy  it  may  be  separated  by  quartation 
with  silver. 

Alloys. — Equal  weights  of  platinum  and  gold  afford 
a  malleable  alloy  ;  the  brilliancy  of  appearance  char- 
acteristic of  gold  is,  however,  much  lessened  by  the 
admixture.  The  two  metals,  combined  in  the  propor- 
tions of  i  part  of  platinum  to  9.5  of  gold,  form  an 
alloy  of  the  same  density  as  platinum.  An  excess  of 
platinum  with  gold  yields  an  alloy  which  is  infusible 
at  ordinary  furnace -heat 

The  tenacity  of  gold  is  very  greatly  increased  by 
admixture  of  platinum,  while  at  the  same  time  it  is 
rendered  more  elastic. 

Platinum  and  silver  may  be  combined  in  all  propor- 
tions, constituting  alloys  of  greater  hardness  than  either 
of  their  constituents,  while  the  color  is  between  the 
color  of  silver  and  that  of  platinum.  Hot  sulfuric 
acid  will  dissolve  the  silver  from  an  alloy  of  this  kind, 
and  when  one  part  of  platinum  is  alloyed  with  ten 
parts  of  silver  both  metals  may  be  dissolved  by  nitric 
acid. 

Platinum  and  mercury  do  not  amalgamate  readily, 
and  combination  can  only  be  effected  by  rubbing 
finely- divided  platinum,  such  as  is  reduced  from  the 
ammonio-chlorid,    in    a  heated  mortar  with    mercury 


PLATINUM.  195 

moistened  with  water  acidulated  with  acetic  acid.  By 
this  means  an  unctuous  amalgam  is  obtained,  which 
has  been  employed  in  platinizing  metallic  objects  in  a 
manner  similar  to  that  known  as  fire-gilding. 

The  use  of  platinum  as  a  constituent  in  alloys  for 
dental  amalgams  has  been  almost  entirely  abandoned. 
The  author  found,  as  the  result  of  a  large  number  of 
experiments,  that  it  rendered  the  alloy  very  brittle  ; 
and,  while  its  presence  seemed  to  retard  amalgama- 
tion, it  increased  the  capacity  of  the  alloy  for  mercury 
(see  page  54). 

Iridium  confers  upon  platinum  great  hardness  and 
tenacity  ;  indeed,  the  alloy  resulting  from  this  combi- 
nation is  so  rigid  that  it  is  with  the  greatest  difficulty 
it  can  be  swaged  into  plates.  It  is,  nevertheless,  an 
alloy  of  great  value  to  the  mechanical  dentist,  as  it 
affords  a  means  of  obtaining  greater  strength  in  arti- 
ficial dentures  of  the  "continuous-gum"  class,  and  it 
has  been  used  in  the  author's  laboratory  since  1870  in 
connection  with  vulcanized  rubber,  the  plate  being 
constructed  of  iridio-platinum,  with  the  teeth,  single 
or  in  sections,  attached  by  means  of  rubber.  The 
swaging  requires  the  use  of  the  zinc  counter-die,  and 
when  the  ridge  is  very  prominent  it  is  best  not  to 
attempt  to  carry  the  plate  entirely  over  it,  rather  allow- 
ing the  rubber  to  take  its  place.  An  artificial  denture 
constructed  in  this  way  has  no  superior  in  point  of 
strength  and  durability. 

Nothing  but  pure  gold  should  be  used  as  a  solder  in 
uniting  two  pieces  of  platinum  or  iridio-platinum. 
Indeed,  so  feeble  is  the  union  between  the  latter  and 
an  ordinary  gold  solder  that  two  pieces  united  by  its 
agency  may  be  readily  torn  apart  with  the  pliers.     The 


I96  DENTAL    METALLURGY. 

addition  of  iridium  is  also  of  value  in  the  construction 
of  platinum  vessels  for  experimental  laboratory  use, 
as  the  metal  is  thereby  rendered  more  resistant  to  high 
temperatures,  and  less  susceptible  to  the  action  of 
chemicals. 

Platinum  combines  with  tin  in  all  proportions,  and 
the  resulting  alloy  is  hard,  brittle,  and  more  or  less 
fusible.  Between  the  platinoid  metals  and  tin,  at  the 
moment  of  fusing  together,  phenomena  very  suggest- 
ive of  true  chemical  union  have  been  observed,  and 
if  tin  and  platinum  foils  be  rolled  together  and  heated 
under  the  blow-pipe,  combination  takes  place  explo- 
sively. It  is  in  consequence  of  this  affinity  that  two 
such  metals,  one  of  which  is  infusible  at  ordinary 
furnace-temperature,  while  the  other  is  readily  fusible 
at  a  low  degree  of  heat,  may,  with  the  greatest  facility, 
be  melted  together  to  form  an  alloy.* 

Oxids. — Platinum  unites  with  oxygen  to  form  two 
compounds, — the  monoxid  or  platinous  oxid  (PtO), 
and  the  dioxid  or  platinic  oxid  (Pt02).  The  first  is 
obtained  as  a  black  powder  by  digesting  the  dichlorid 
with  caustic  potash.  The  second  (PtOa)  may  be  pre- 
pared by  adding  barium  nitrate  to  a  solution  of  platinic 
sulfate.  Barium  sulfate  and  platinic  nitrate  are  thus 
formed,  and  from  the  latter  caustic  soda  precipitates 
one-half  of  the  platinum  as  platinic  hydrate,  a  bulky 
brown  powder,  which,  when  gently  heated,  becomes 
black  and  anhydrous.  It  is  also  formed  when  platinic 
chlorid  is  boiled  with  an  excess  of  caustic  soda,  and 
acetic  acid  added.  It  combines  with  bases  and  dis- 
solves in  acids.      Platinic  oxid  with  ammonia  forms 

*  See  chapter  on  "Alloys." 


PLATINUM.  197 

an  explosive  compound,  which  detonates  violently  at 
about  4000  F.  Both  oxids  of  platinum  are  reduced 
to  the  metallic  state  by  heating  to  redness. 

Platinic  chlorid  (PtCl4)  is  the  most  useful  salt  of  the 
metal,  and  is  the  one  from  which  all  the  platinum  com- 
pounds are  obtained.  It  may  be  prepared  by  dis- 
solving scraps  of  platinum  in  a  mixture  of  four  meas- 
ures of  hydrochloric  acid  with  one  of  nitric  acid,  one 
hundred  grains  of  platinum  requiring  the  presence  of 
two  ounces  of  hydrochloric  acid.  After  complete 
solution  the  liquid  is  evaporated  at  a  gentle  heat  to  a 
syrupy  consistence,  redissolved  in  hydrochloric  acid, 
and  again  evaporated  to  expel  excess  of  nitric  acid. 
The  syrup-like  fluid  solidifies  on  cooling  to  a  red- 
brown  mass,  which  is  deliquescent  and  readily  dissolves 
in  water  or  alcohol. 

Spongy  platinum  is  prepared  by  heating  the  yellow 
crystalline  precipitate  obtained  by  the  addition  of 
ammonic  chlorid. 

Platinous  chlorid  (PtCl2)  may  be  formed  by  heating 
platinic  chlorid  to  a  point  somewhat  above  4500  F.  A 
very  high  temperature  reduces  it  to  the  metallic  state. 

Sulfids. — The  compounds  PtS  and  PtS,  are  pro- 
duced by  the  action  of  hydrogen  sulfid,  or  the  hydro- 
sulfid  of  an  alkali  metal,  on  the  dichlorid  and  tetra- 
chlorid  of  platinum  respectively.  They  are  both  black, 
insoluble  substances. 

Discrimination  of  Platinum  Salts.  —  1st.  A  blackish- 
brown  precipitate,  insoluble  in  nitric  or  hydrochloric 
acid  singly,  will  be  thrown  down  by  the  addition  of 
hydrogen  sulfid  (H,S). 

2d.  Ammonia  or  potash  throws  down  a  yellow 
crystalline  precipitate. 


198  DENTAL     METALLURGY. 

3d.  A  brown  hydrated  platinic  oxid  is  precipitated 
from  the  salts  of  platinum  by  the  addition  of  soda, 
and  it  should  be  remembered  that  the  precipitate  is 
soluble  in  an  excess  of  the  soda. 

4th.  A  deep-brown  color  is  imparted  to  solutions  of 
platinum  salts  by  the  addition  of  stannous  chlorid,  but 
no  precipitate  is  obtained. 

Quantitatively,  platinum  may  be  separated  from 
other  metals  with  which  it  is  likely  to  be  associated  by 
precipitating  with  ammonium  chlorid.  This  is  added 
to  the  platinum  solution,  followed  by  a  little  alcohol. 
The  precipitate  is  collected,  washed  with  alcohol,  and 
dried,  when  it  is  ready  for  weighing.  Every  100  parts 
will  contain  44.28  of  platinum. 


CHAPTER  XI. 

IRIDIUM. 
Atomic  Weight,   198.     Symbol,   Ir. 

IRIDIUM,  named  from  Iris,  the  rainbow,  because 
of  the  varied  colors  of  its  compounds,  has  already 
been  mentioned  as  occurring  in  the  insoluble  alloy 
from  the  platinum  ores,  and  disseminated  in  small, 
hard  points  throughout  the  substance  of  California 
gold.  It  is  also  obtained  when  crude  platinum  is  dis- 
solved in  nitro-muriatic  acid.  A  gray,  scaly,  metallic 
substance  is  found  at  the  bottom  of  the  vessel,  which 
has  entirely  resisted  the  action  of  the  acid.  This  is 
osmiridium,  a  native  alloy  of  iridium  and  osmium. 
The  subsequent  treatment  for  the  separation  of  the 
two  metals  consists  in  reducing  them  to  powder,  com- 
bining them  with  an  equal  weight  of  dry  sodium 
chlorid,  and  heating  to  redness  in  a  glass  tube,  through 
which  moist  chlorin  gas  is  allowed  to  pass.  The  tube 
is  connected  with  a  receiver  containing  a  solution  of 
ammonia.  The  gas  is  quickly  absorbed,  and  iridium 
chlorid  and  osmium  chlorid  are  formed.  The  first 
remains  combined  with  the  sodium  chlorid,  while  the 
osmium  chlorid,  being  a  volatile  substance,  passes  into 
the  receiver.  The  contents  of  the  tube,  consisting  of 
iridium  and  sodium  chlorids,  when  cold,  are  dissolved 
with  water,  and  mixed  with  an  excess  of  sodium  car- 
bonate and  evaporated  to  dryness.  After  ignition  in 
a  crucible,  boiling  in  water,  and  drying,  the  metal  may 
be  reduced  by  hydrogen  (at  a  high  temperature),  and 

199 


200  DENTAL   METALLURGY. 

heating-  successively  with  water  and  strong  hydro- 
chloric acid  to  free  it  of  the  alkali  and  iron.  What 
remains  consists  of  metallic  iridium  in  a  finely- divided 
state. 

Properties. — Iridium  is  an  exceedingly  hard  and 
brittle  metal,  nearly  white  in  color,  and  fusible  only 
by  the  oxyhydrogen  blow-pipe,  by  which  means  it 
has  been  converted  into  a  white  mass  having  some- 
what the  appearance  of  polished  steel.  It  is  hard^ 
and  brittle  while  cold,  but  it  is  rendered  somewhat 
malleable  at  red  heat.  Its  density  is  about  the  same 
as  that  of  platinum.  It  has  been  obtained  in  tolerably 
compact  masses  by  compressing  some  of  the  metal  in 
a  very  fine  state  and  then  heating  to  the  highest  point 
attainable  in  a  forge-fire.  The  density  of  a  sample  of 
iridium  prepared  in  this  way  will  not  exceed  16.0. 

When  reduced  by  hydrogen  at  low  temperatures  it 
dissolves  in  nitro-hydrochloric  acid,  but  is  rendered 
insoluble  in  all  acids  by  exposure  to  white  heat.  It 
may  again  be  dissolved  by  igniting  it  with  a  mixture 
of  the  chlorid  of  potassium  and  sodium  in  a  current 
of  chlorin. 

Mr.  John  Holland,  of  Cincinnati,  devised  an  in- 
genious process  for  the  preparation  of  larger  pieces 
of  iridium  than  are  generally  found  in  nature.  Some 
of  the  ore  is  heated  to  whiteness  with  phosphorus  in 
a  Hessian  crucible,  by  which  means  complete  fusion 
is  obtained.  Phosphor-iridium,  as  this  compound  may 
be  called,  is  as  hard  as  the  iridosmine  from  which  it  is 

*  Its  hardness  is  such  that  the  hardest  file  will  make  no  impression  on 
it.  In  working  California  gold,  which  often  contains  disseminated  through 
it  small  grains  of  the  native  alloy  of  osmium  and  iridium,  the  file  is  sensi- 
bly injured  by  contact  with  it,  while  in  coining  operations  much  incon- 
venience and  injury  are  caused  by  its  presence. 


IRIDIUM.  20I 

prepared,    and   is    used   for   making   points   for    the 
Mackinnon  stylographic  pen. 

Alloys. — Platinum  containing  a  small  quantity  of 
iridium  is  rendered  more  rigid,  and  is  of  great  value 
as  a  means  of  strengthening  continuous-gum  work, 
by  forming  the  backing  of  it,  especially  in  partial 
lower  sets,  where,  in  addition  to  the  two  pieces  of 
pure  platinum  covering  the  gums  back  of  the  natural 
(front)  teeth,  an  extra  piece  of  platinum  alloyed  with 
iridium,  No.  26,  may  be  used.  It  also  answers  well 
in  combination  with  vulcanizable  rubber, *  for  either 
entire  or  partial  cases,  and  though  decidedly  the  most 
refractory  of  the  various  alloys  employed  in  the  dental 
laboratory,  it  may  be  perfectly  swaged  by  the  use  of 
a  zinc  counter-die.  In  the  construction  of  partial 
cases  it  may  be  employed  for  clasps,  but  wherever  it 
is  necessary  to  unite  two  pieces  by  soldering  nothing 
but  pure  gold  should  be  employed  as  the  solder. 
Ordinary  gold  solders  do  not  afford  a  strong  union. 

Iridium  unites  with  oxygen,  sulfur,  chlorin,  and 
iodin,  to  form  oxids,  sulfids,  chlorids,  and  iodids. 

DiscriminaMon. — With  ammonium  or  potassium 
chlorid,  iridium  solutions  afford  a  dark  reddish- 
brown  crystalline  precipitate  of  ammonium  or  chlor- 
idium,  the  color  of  which,  and  the  fact  that  it  is 
reducible  to  soluble  chlor-iridium  when  treated  with 
hydrogen  sulfid,  distinguishes  it  from  the  correspond- 
ing platinum  precipitate. 

*  See  chapter  on  "  Platinum." 


14 


CHAPTER   XII. 

PALLADIUM. 

Atomic  Weight,  106.5.    Symbol,  Pd. 

PALLADIUM,  rhodium,  iridium,  ruthenium,  os- 
mium, and  davyum*  constitute  a  group  of  metals 
which  possess  many  properties  in  common. 
They  are  also  closely  allied  by  natural  association. 

Crude  platinum  is  a  native  alloy  of  platinum  with 
these  metals,  and  is  the  source  whence  they  are  usu- 
ally obtained. f  Palladium  is,  however,  occasionally 
found  native,  in  a  comparatively  pure  state,  intermixed 
with  platinum,  from  which  metal  it  may  be  readily 
distinguished  by  the  fibrous  appearance  of  its  grains. 

It  is  obtained  at  the  present  time  chiefly  from  the 
solution  of  crude  platinum,  after  that  metal  has  been 
separated  by  precipitation  with  ammonic  chlorid. 
The  remaining  liquid  is  neutralized  by  sodium  car- 
bonate, and  mixed  with  a  solution  of  mercuric  cyanid  ; 
palladium  cyanid  separates  as  a  whitish,  insoluble 
substance,  which,  on  being  washed,  dried,  and  heated 

*  This  metal,  named  in  honor  of  Sir  Humphry  Davy,  was  discovered 
by  Kern  in  1877,  m  platiniferous  sand.  It  was  obtained  from  the  mother 
liquors  after  the  separation  of  platinum,  palladium,  osmium,  and  iridium, 
by  heating  them  with  an  excess  of  ammonium  chlorid  and  nitrate.  A 
deep-red  precipitate  was  obtained,  which  after  calcination  at  a  red  heat 
left  a  grayish  mass  resembling  platinum  sponge.  This,  when  fused  by  the 
oxyhydrogen  blow-pipe,  furnished  a  silver-white  metal  of  great  hardness, 
though  malleable  at  a  red  heat,  having  a  density  of  9.39;  soluble  in  aqua 
regia,  but  only  slightly  acted  upon  by  boiling  sulfuric  acid. 

f  Palladium  was,  at  one  time,  obtained  in  considerable  quantities  from 
Brazilian  gold,  with  which  it  was  associated  as  an  alloy,  but  this  means 
of  supply  having  failed,  it  has  become  too  expensive  for  employment  in 
the  industrial  arts. 

202 


PALLADIUM.  203 

to  redness,  yields  metallic  palladium  in  a  spongy 
state,  which  admits  of  welding  into  a  solid  mass  in 
the  same  manner  as  platinum. 

In  appearance  palladium  resembles  an  alloy  of 
platinum  and  gold  wherein  the  proportion  of  the 
former  greatly  exceeds  that  of  the  latter.  It  possesses 
the  qualities  of  malleability  and  ductility,  but  in  those 
properties  it  is  probably  inferior  to  platinum.  Its 
density  differs  very  much  from  platinum,  being  only 
11.8.  It  is  more  oxidizable  than  platinum,  and  when 
heated  to  redness  in  the  air,  especially  when  in  a 
finely-divided  state,  it  acquires  a  superficial  film  of 
oxid  of  a  bluish  color,  which  may  be  again  reduced  at 
a  high  temperature.  It  does  not,  however,  oxidize 
in  the  air  unless  its  temperature  is  raised  to  red  heat, 
and  on  account  of  its  unalterability  in  the  air  and  its 
bright  silver-white  color,  which  is  not  affected  by  ex- 
posure to  sulfuretted  hydrogen,  it  is  used  for  prepar- 
ing the  graduated  surfaces  of  astronomical  instruments 
and  for  coating  silver  goods.  It  is  the  most  fusible  of 
the  platinum  metals,  and  requires  about  the  heat 
needed  to  fuse  malleable  iron  to  reduce  it  to  a  state  of 
fluidity,  in  which  condition  it  absorbs  oxygen,  parting 
with  it  again  in  cooling  in  the  same  way  that  silver 
does.  It  is  dissolved  by  nitric  acid,  but  its  best  sol- 
vent is  nitro-hydrochloric  acid.  It  is  attacked  by 
iodin,  and  may  be  distinguished  from  platinum  by 
heating  a  drop  of  tincture  of  iodin  upon  it,  when  it 
will  show  a  stain,  while  platinum  similarly  treated  is 
quite  unaffected. 

Alloys. — In  a  finely-divided  state  it  unites  readily 
with  mercury,  and  there  would  appear  to  be  some 
chemical  affinity  between   the  two,   the  union  being 


204  DENTAL   METALLURGY. 

accompanied  by  evolution  of  heat.  As  the  amalgam 
cools  it  sets  and  becomes  tolerably  hard,  and  it  is 
stated  that  it  expands  in  hardening.* 

Palladium  Precipitate. — The  form  in  which  t  he 
metal  is  generally  used  in  amalgams  seems  to  lose  its 
affinity  for  mercury  after  long  exposure  to  the  air, 
and  combination  can  only  be  effected  by  the  assistance 
of  gentle  heat.  Apparatus  for  determining  the  expan- 
sion or  contraction  of  amalgams  has  been  employed 
to  test  the  expansibility  of  palladium  amalgams.  One 
of  these,  consisting  of  a  trough  having  a  movable  end 
with  a  screw  micrometer  capable  of  noting  the  most 
minute  change,  indicated  in  one  specimen  of  palladium 
amalgam  an  expansion  of  i  in  25f  of  its  diameter, 
though  the  amount  of  solid  matter  in  this  class  of 
amalgams  is  very  small,  as  it  requires  three  parts  of 
mercury  to  one  of  precipitated  palladium. 

Mr.  Coleman  J  states  that  palladium  amalgam  sets 
very  rapidly,  and  when  mixed  in  large  enough  quan- 
tities to  fill  good-sized  cavities  all  the  phenomena  of 
true  chemical  affinity  are  observed.  He  also  gives  the 
following  description  of  the  manner  of  mixing  and 
applying  the  amalgam  :  '  'About  as  much  mercury  as 
would  fill  the  cavity  to  be  treated  is  placed  in  the  palm 
of  the  hand,  and  the  palladium  powder  very  gradually 
added.  It  requires  some  careful  rubbing  with  the  fore- 
finger before  the  two  become  incorporated,  when  it 
should  be  divided  into  smallish  pellets,  and  these  rapidly 
carried,  one  after  another,  to  the  cavity,  each  piece 

*  Hitchcock  on  Dental  Amalgams,  page  33,  Transactions  of  the  New 
York  Odontological  Society,  1874. 
f  British  Journal  of  Dental  Science,  Vol.  IX,  Part  II,  July,  1888. 
\  Manual  of  Dental  Surgery  and  Pathology,  page  129. 


PALLADIUM.  205 

being-  well  compressed,  and  rubbed  into  the  irregu- 
larities of  its  walls  with  a  burnisher  or  compressing 
instrument."  Mr.  Coleman  further  states  that  this  is 
probably  the  most  durable  of  all  the  amalgams,  but 
the  most  difficult  to  manipulate.  Its  surface  changes 
to  a  black  color,  but,  as  a  rule,  it  does  not  stain  the 
structure  of  the  tooth. 

Palladium  has  been  used  as  a  constituent  in  dental 
alloys  (amalgams),  and  when  added  to  a  gold,  silver, 
and  tin  alloy  it  probably  has  about  the  same  effect  as 
does  platinum.  It  has  been  observed,  however,  that 
dental  alloys  in  which  it  is  a  constituent  blacken  to  a 
greater  extent  than  when  it  is  omitted.  Mr.  Fletcher,  * 
after  a  series  of  experiments,  finally  abandoned  its  use 
in  dental  amalgams. 

Silver  and  palladium  unite  in  all  proportions,  form- 
ing alloys  which  retain  an  exceedingly  brilliant  sur- 
face. 

Gold  and  palladium,  in  equal  proportions,  form  a 
hard,  gray  alloy.  Indeed,  all  the  alloys  formed  of 
these  two  metals  are  exceedingly  hard. 

Palladium  and  platinum  form  a  hard  alloy,  which 
fuses  below  the  melting-point  of  palladium. 

Palladium  combines  with  antimony,  bismuth,  zinc, 
tin,  iron,  and  lead,  forming  brittle  alloys.  With 
nickel  it  forms  malleable  alloys  which  are  susceptible 
of  high  polish. 

It  is  quite  probable  that  palladium,  either  alone  or 
alloyed  with  other  metals,  might  be  advantageously 
employed  in  prosthetic  dentistry,  but  its  high  price 
practically  excludes  it  from  the  dentist's  laboratory. 

Palladium    enters    into   combination    with    chlorin 

*See  chapter  on  "Amalgams." 


206  DENTAL     METALLURGY. 

(PdCL),  oxygen  (PdO  and  Pd02),  and  sulfur  (PdS). 
When  heated  to  redness  in  a  stream  of  hydrogen 
gas,  it  possesses  the  remarkable  power  of  absorbing 
about  643  times  its  volume  of  the  gas,  whereby  its 
specific  gravity  is  reduced  to  11.06.  The  absorption 
of  the  gas  is  attended  with  evolution  of  heat,  and  this 
fact,  together  with  the  supposed  constancy  of  com- 
position of  the  hydrogenized  palladium,  led  Graham 
to  suppose  that  the  product  was  a  definite  chemical 
compound,  to  which  he  assigned  the  formula  PdH2. 
Troost  and  Hautefeuille,  however,  found  the  correct 
composition  to  be  Pd2H  * 

Discrimination. — Mercuric  cyanid  is  the  chief  test 
for  palladium. f  It  precipitates  a  yellowish-white 
cyanid,  soluble  in  hydrochloric  acid,  which  is  easily 
reduced  by  heat  to  metallic  palladium,  when  it  is 
ready  for  weighing. 

*  Watt's  Dictionary  of  Chemistry. 

f  There  are  other  reagents  employed  in  the  discrimination  of  palladium, 
— sulfuretted  hydrogen,  potash,  ammonia  and  its  carbonates,  ferrous 
sulfate,  and  stannous  chlorid.  Mercuric  cyanid,  however,  is  the  one 
generally  employed  in  the  quantitative  estimation  of  palladium. 


CHAPTER    XIII. 

IRON. 

Atomic   Weight,   56.     Symbol,   Fe  (Ferrum). 

IRON  is  present  in  nearly  all  forms  of  rock,  clay, 
sand,  and  earth.  It  is  the  most  widely  diffused 
of  the  natural  coloring  ingredients,  and  its  presence 
may  be  readily  distinguished  by  the  color  which  it 
imparts.  It  is  found  in  varying  proportions  in  plants 
and  the  bodies  of  animals,  the  blood  of  the  latter  con- 
taining about  0.5  per  cent,  of  iron  associated  with  its 
coloring-matter. 

Iron  seems  to  have  been  known  very  early  in  the 
world's  history,  and  at  a  remote  period  instruments  of 
agriculture  and  war  were  manufactured  of  it.'*  Its 
chief  ores  are  oxids,  carbonate,  and  sulfids.  Metallic 
iront  is  met  with  in  nature  in  the  meteorites  or  metallic 
masses  of  unknown  origin  which  occasionally  fall  to 
the  earth.  J  The  carbonates  and  oxids  are  the  ores  from 
which  iron  is  chiefly  obtained.  Their  reduction — the 
oxids  especially — is  exceedingly  simple,  and  consists 
in  merely  heating  them  in  contact  with  carbonaceous 
compounds,  by  which  means  the  metal  is  liberated. 

*  Archaeologists  distinguish  a  Bronze  Age  in  prehistoric  times  inter- 
mediate between  those  of  Stone  and  Iron. 

I  Metallic  iron,  though  of  exceedingly  rare  occurrence,  has  been  found 
at  Canaan,  in  Connecticut,  forming  a  vein  about  two  inches  thick,  in  mica 
slate. 

I  Isolated  masses  of  soft  iron,  sometimes  of  large  dimensions,  have  been 
found  upon  the  surface  of  the  earth  in  South  America  ;  they  are  supposed 
to  have  had  a  similar  origin. 

207 


208  DENTAL   METALLURGY. 

Properties. — Pure  iron  is  white  in  color,  extremely- 
soft  and  tough,  and  has  a  specific  gravity  of  7. 8.  Iron 
may  be  regarded  as  possessing  a  greater  number  of 
valuable  qualities  than  any  other  metal ;  hence  it 
occupies  the  highest  place  in  the  useful  arts.  Although 
possessing  nearly  twice  as  much  strength  as  the 
strongest  of  the  other  metals,  it  is  yet  one  of  the 
lightest,  and  is  therefore  peculiarly  fitted  for  use  in 
the  construction  of  bridges,  ships,  etc.  It  is  rendered 
so  ductile  by  heating  that  it  may  be  rolled  into  very 
thin  sheets  or  drawn  into  the  finest  wire  ;  and  yet  at 
ordinary  temperatures  it  is  the  least  yielding  of  the 
metals  in  common  use,  and  may  always  be  relied  upon 
to  afford  a  rigid  support.  An  iron  wire  one-tenth  of 
an  inch  in  diameter  is  capable  of  sustaining  seven  hun- 
dred and  five  pounds.  It  is  very  difficult  of  fusion, 
and  before  becoming  liquid  passes  through  a  soft  or 
pasty  condition.  Pieces  of  iron  pressed  or  hammered 
while  in  this  state  cohere  or  weld  together. 

The  fusing-point  of  iron  has  been  estimated  at  29000 
F.  It  is  soluble  in  nitric,  dilute  sulfuric,  and  hydro- 
chloric acids,  but  is  not  much  affected  by  strong  sul- 
furic. Chlorin,  iodin,  and  bromin  attack  it  readily. 
Under  certain  circumstances  it  is  not  acted  upon  by 
strong  nitric  acid.  If  a  piece  of  platinum  wire  be 
kept  in  contact  with  it,  it  will  remain  in  this  acid  for 
many  weeks  without  being  acted  upon.  Its  crystalline 
form  is  supposed  to  be  a  cube.  When  rolled  into  bars 
or  drawn  into  wire  it  possesses  a  fibrous  texture,  upon 
the  perfection  of  which  much  of  its  strength  and  value 
depend.  It  is  the  most  tenacious  of  all  the  metals. 
At  red  heat  iron  decomposes  water  evolving  hydro- 
gen, and  is  changed  into    the    black  oxid.     It  is  a 


IRON.  209 

strongly  magnetic  metal,  but  loses  this  quality  when 
heated  to  redness. 

Iron  does  not  oxidize  in  dry  air  at  ordinary  tem- 
peratures, and  it  may  be  immersed  in  water  from 
which  the  air  has  been  carefully  excluded  without 
change.  Contact  with  a  more  electro-positive  metal 
will  also  prevent  oxidation.  Thus,  fine  steel  instru- 
ments are  sometimes  packed  for  exportation  by  wrap- 
ping in  thin  sheet  zinc.  For  a  description  of  the 
compounds  of  iron  and  the  reagents  employed  in  its 
discrimination,  the  student  is  referred  to  Fownes's 
"  Elementary  Chemistry." 

The  value  of  iron  does  not  depend  alone  upon  its 
physical  properties,  for  it  enters  into  a  large  number 
of  compounds  which  are  of  great  use  in  the  arts,  and 
its  chemical  relation  to  carbon  is  such  that  the  addi- 
tion of  a  small  quantity  of  that  element  converts  it 
into  steel,  harder  and  more  elastic  than  iron,  while 
a  larger  quantity  of  carbon  produces  cast  iron,  which 
is  so  fusible  that  many  useful  articles  may  be  made 
of  it  by  casting. 

Steel. — Herodotus  states  that  among  the  most 
precious  gifts  presented  by  the  Indian  monarch  Porus 
to  Alexander  the  Great  was  a  pound  of  steel,  the 
value  of  which  at  that  time  has  been  estimated  at 
about  two  hundred  dollars.  At  a  later  period  the 
manufacture  of  steel  in  its  application  to  warlike 
instruments  was  carried  to  a  great  state  of  perfection 
in  India  and  in  the  south  of  Europe. 

Steel  differs  from  iron  in  possessing  the  property 
of  becoming  very  hard  and  brittle,  if,  when  heated  to 
bright  redness,  it  is  suddenly  cooled  by  being  plunged 
into  water.     Steel  is  simply  iron  chemically  combined 


2IO  DENTAL     METALLURGY. 

with  the  precise  amount  of  carbon  which  will  produce 
the  condition  referred  to,  together  with  additional 
toughness.  It  does  not,  however,  become  decidedly 
steel-like  until  the  carbon  amounts  to  0.3  per  cent. 
The  hardest  steel  contains  about  1.2  per  cent,  of  car- 
bon, and  when  that  proportion  is  exceeded  it  begins 
to  assume  the  properties  of  cast  iron. 

There  are  several  processes  by  which  steel  may  be 
produced.  Bars  of  iron  imbedded  in  charcoal  powder 
in  a  suitable  crucible  or  chest  made  of  some  substance 
capable  of  resisting  the  fire  are,  after  several  hours' 
exposure  to  heat,  converted  into  steel,  the  iron  taking 
up  the  requisite  amount  of  carbon.  The  product  of 
this  operation  is  called  blistered  steel,  and  is  far  from 
uniform,  either  in  composition  or  texture,  as  portions 
of  the  bars  thus  produced  will  be  found  to  contain 
more  carbon  than  others,  and  the  interior  to  be  more 
or  less  porous.  For  the  purpose  of  improving  its 
quality  the  bars  are  cut  into  short  lengths,  made  up 
into  bundles,  heated  to  the  welding-point,  and  placed 
under  a  powerful  tilt-hammer,  which  consolidates  each 
bundle  into  one  mass.     This  is  called  shear  steel. 

Fusing  and  casting  steel  is  another  process  for  the 
treatment  of  the  blistered  form,  by  which  is  produced 
the  best  and  most  homogeneous  variety.  It  consists 
in  fusing  about  thirty  pounds  of  broken  fragments  of 
blistered  steel  in  a  plumbago  crucible,  the  surface 
being  protected  from  oxidation  by  glass  melted  upon 
it.  When  perfectly  fluid  the  steel  is  cast  into  ingots, 
and  when  it  is  desirable  to  form  a  very  large  ingot, 
several  crucibles  are  simultaneously  emptied  into  the 
same  mold.  Cast  steel  is  superior  in  density  and 
hardness  to  shear  steel,  and  is  the  form  best  adapted 


IRON.  211 

to  the  manufacture  of  fine  cutting-instruments.  It  is, 
however,  somewhat  brittle  at  red  heat,  and  much 
care  and  skill  are  required  in  forging  it.  The  addition 
to  it  while  fused  of  one  part  of  a  mixture  of  charcoal 
and  oxid  of  manganese  affords  a  fine-grained  steel, 
which  may  be  cast  into  a  bar  of  wrought  iron  in  the 
ingot-mold,  in  order  that  the  tenacity  of  the  iron  may 
be  an  offset  to  the  brittleness  of  the  steel  when  forged 
together,  while  it  affords  an  economical  compound  in 
the  manufacture  of  cutting-implements,  the  iron  form- 
ing the  back  and  the  steel  the  edge  of  the  instrument. 

Bessemer*  steel  is  produced  by  forcing  atmospheric 
air  into  melted  cast  iron.  The  carbon,  which  is 
oxidized  more  readily  than  the  iron,  escapes  in  the 
form  of  carbon  monoxid,  combustion  of  which  takes 
place  on  coming  in  contact  with  atmospheric  air, 
and  sufficient  heat  is  thus  generated  to  keep  the  tem- 
perature above  the  melting-point  of  steel  during  the 
operation.  The  current  of  air  is  stopped  as  soon  as 
the  decarburation  has  progressed  far  enough,  when 
a  quantity  of  white  pig-iron  containing  manganese  is 
added  to  the  fluid  metal  for  the  purpose  of  assisting 
the  separation  of  gas  from  the  melted  metal.  It  is 
then  ready  for  casting. 

Some  New  Iron  Alloys. — The  combinations  of  iron 
with  aluminum,  chromium,  copper,  manganese, 
nickel,  silicon,  and  tungsten  constitute  a  class  of  alloys 
which  are  essentially  new  and  are  termed  steels,  and 
are  usually  designated  with  a  prefix  of  the  name  of 
the  particular  element  present,  as  nickel  steel,  chrome 
steel,  etc. 

*  For  other  methods  of  producing  steel,  see    Percey's,    Phillips's,  or 
Makins's  works  on  Metallurgy. 


212  DENTAL   METALLURGY. 

Aluminum  Steel. — The  addition  of  aluminum 
slightly  increases  the  tensile  strength,  and  propor- 
tionately the  elastic  limit,  in  rolled  and  cast  steel  when 
the  amount  added  is  not  greater  than  one  per  cent. 

Chrome  Steel. — Chromium  increases  the  hardness, 
tensile  strength,  and  elastic  limit  of  iron,  but  lessens 
its  weldability.  Ferro- chrome  is  made  by  heating 
the  mixed  oxids  of  iron  and  chromium  in  brasqued 
crucibles,  adding  powdered  charcoal  and  fluxes. 
Chrome  steel  is  then  produced  from  ferro-chrome  by 
melting  it  with  wrought  iron  or  steel  in  graphite  cru- 
cibles. It  has  been  stated  that  the  presence  of  chro- 
mium in  steel  renders  it  more  susceptible  to  oxida- 
tion by  exposure  to  air  and  moisture  than  ordinary 
steel.  It  was  thought  that  chrome  steel  would  ad- 
mirably fill  certain  special  requirements  where  great 
hardness  and  toughness  is  needed,  as  the  manufacture 
of  dental  instruments,  projectiles,  cuirasses,  etc.,  but 
there  are  other  steel  alloys  coming  into  use  which 
are  so  much  better  that  it  is  probably  only  a  ques- 
tion of  time  when  it  will  be  superseded. 

Copper  Steel. — M.  Henry  Schneider,  of  Creusot, 
France,  obtained  patents  for  the  manufacture  of 
alloys  of  iron  and  copper  and  steel  and  copper.  This 
alloy  can  be  made  in  crucible,  cupola,  or  open-hearth 
furnace.  The  furnace  is  charged  with  copper  scraps 
and  cast  iron  mixed  between  layers  of  coke,  or  if  a 
cupreous  coke  be  employed  then  the  cast  iron  is  laid  in 
alternate  layers  with  it,  and  a  layer  of  anthracite  is  laid 
over  the  whole.  The  alloy  thus  formed  contains  gener- 
ally from  five  to  twenty  per  cent,  of  copper,  according 
to  the  purpose  for  which  it  is  to  be  employed,  and  it  is 
remarkable  for  its  great  strength,  tenacity,  and  malle- 


IRON.  213 

ability,  properties  which  may  still  further  be  developed 
by  chilling  or  tempering. 

Nickel  Steel. — United  States  patents  415,657  and 
415,655,  November  19,  1889,  were  granted  M.  Henry 
Schneider,  of  Creusot,  France,  for  the  manufacture  of 
alloys  of  cast  iron  and  nickel  and  steel  and  nickel  re- 
spectively. The  alloy  of  cast  iron  and  nickel  con- 
tains from  five  to  thirty  per  cent,  of  nickel,  and  is  re- 
markable for  its  great  elasticity  and  strength,  proper- 
ties which  admit  of  further  development  by  the  usual 
chilling  or  tempering. 

The  alloy  of  steel  and  nickel  usually  contains  about 
five  per  cent,  nickel,  and  is  especially  suitable  for  use 
in  the  construction  of  ordnance,  armor-plate,  gun- 
barrels,  and  projectiles.  Some  specimens  of  nickel 
steel  recently  produced  by  Carnegie,  Phipps  &  Co., 
for  the  U.  S.  Navy  Department,  containing  three- 
sixteenths  per  cent,  nickel,  showed,  when  tested,  the 
following  results :  Elastic  limit,  59,000  and  60,000 
pounds  per  square  inch  ;  ultimate  tensile  strength, 
100,000  and  102,000  pounds  per  square  inch. 

It  is  stated  that  the  presence  of  manganese  in  nickel 
steel  is  most  important,  as  it  appears  that  without  the 
aid  of  manganese  in  proper  proportions  the  best  re- 
sults could  not  be  obtained.  Nickel  steel  is  said  to  be 
less  liable  to  corrode  in  salt  water  than  ordinary  steel, 
which,  it  maybe  proper  to  observe  in  this  connection, 
is  more  readily  acted  upon  by  sea-water  than  are  the 
more  impure  grades  of  iron. 

The  success  of  the  nickel-steel  armor-plate  at  the 
recent  test  by  the  U.  S.  Navy  Department,  in  which 
the  nickel-steel  plates  alone  withstood  the  eight-inch 
chrome-steel  projectiles  without  cracking,  has  had  a 


214  DENTAL   METALLURGY. 

most  important  influence  upon  the  manufacture  of  that 
alloy. 

Manganese  Steel.  —  The  maximum  of  strength, 
toughness,  and  hardness  is  probably  reached  in  this 
alloy,  when  about  fifteen  per  cent,  of  manganese  is 
added  to  steel.  The  chief  obstacle  to  the  commercial 
production  of  manganese  steel  is  its  extreme  hard- 
ness. The  working  of  some  of  the  grades  of  this  ma- 
terial by  the  ordinary  methods  is  almost  impossible. 

Manganese  steel  is  usually  made  by  adding  ferro- 
manganese  to  molten  Bessemer  or  open-hearth  steel. 
The  extreme  point  of  brittleness  in  this  alloy  occurs 
in  specimens  containing  from  four  to  five  per  cent,  of 
manganese.  Extremes  of  atmosphere,  heat  or  cold, 
do  not  appear  to  affect  the  properties  of  manganese 
steel .  When  a  piece  of  it,  heated  sufficiently  to  be  seen 
red  hot  in  a  dark  room,  is  plunged  into  cold  water,  it 
becomes  soft  enough  to  be  easily  filed.  Hardness  is 
then  restored  by  reheating  to  a  bright  red,  and  cool- 
ing in  air. 

Ha?'dening  and  Tempering. — Hardening  of  ordi- 
nary carbon  steel  is  effected  by  subjecting  the  object 
to  extremes  of  temperature.  The  common  practice 
is  to  first  coat  the  surface  of  the  metal  with  some  car- 
bonaceous substance,  such  as  soap,  to  prevent  scaling 
and  oxidation  of  the  surface.  Ferro-cyanid  of  potas- ' 
sium  has  also  been  used  for  surface-hardening.  This 
salt  contains  cyanogen  (C2N2),  a  gas  consisting  of 
twelve  parts  by  weight  of  carbon  and  fourteen  of 
nitrogen.  This  is  decomposed  at  the  high  tempera- 
ture which  is  employed,  and  supplies  carbon  to  the 
surface  of  the  metal.  This  salt  is,  however,  better 
suited  to  the  process  known  as  case-hardening,  while 


IRON. 


21 


in  re-tempering  dental  instruments  soap  answers  every 
requirement. 

The  metal  is  next  heated  to  the  point  of  full  red- 
ness, and  then  suddenly  plunged  into  cold  water,  oil, 
tallow,  or  mercury,  or,  in  the  case  of  small  objects,  is 
merely  placed  on  a  large  piece  of  cold  metal.  It  is 
thus  rendered  very  hard,  while  at  the  same  time  it 
increases  slightly  in  volume. 

If  hardened  steel  be  heated  to  redness,  and  allowed 
to  cool  slowly,  it  is  again  converted  into  soft  steel, 
but  it  may  be  proportionately  reduced  by  heating  to 
a  temperature  short  of  redness,  the  proper  point  of 
which  may  be  ascertained  by  noting  certain  colors 
which  appear  on  the  ground  or  brightened  surface  of 
a  steel  instrument  when  held  over  a  flame.  This  dis- 
coloration is  due  to  the  formation  of  a  thin  film  of 
oxid,  and  as  the  temperature  rises  the  film  becomes 
thicker  and  darker,  and  the  instrument  softer.  It  is 
therefore  necessary  to  plunge  the  instrument  into  a 
cold  menstruum  the  instant  the  color  indicating  the 
desired  degree  of  hardness  is  reached.  The  follow- 
ing table  indicates  the  tempering  heats  of  various 
instruments  : 


Temperature. 

Color. 

Use. 

4300  to  4500  F. 

Light  yellow. 

Enamel  chisels. 

47o°  F. 

Medium  yellow. 

Excavators. 

4900  F. 

Brown-yellow. 

Pluggers. 

5io°  F. 

Brown-purple. 

Saws,  etc. 

5200  F. 

Purple. 

Wood-cutting  tools. 

5300  to  5700  F. 

Blue. 

When  elasticity  is  desired. 

In  "letting  down"  or  tempering  dental  instruments 
the  flame  of  a  spirit-lamp  may  be  employed,  the  in- 


2l6  DENTAL    METALLURGY. 

strument  being  placed  in  it ;  the  flame  should  strike, 
however,  some  distance  from  the  cutting-end,  and 
when  the  proper  color  reaches  the  end  it  should  be 
thrust  into  water.  Another  very  convenient  means 
of  effecting  the  same  result  consists  in  heating  an  iron 
bar  to  redness  at  one  end,  and  then  fixing  it  in  a  vise. 
The  object  to  be  tempered  is  placed  in  contact  with 
this  until  the  desired  tint  appears. 

Steel  when  fractured  shows  a  fine  silky  appearance 
of  the  broken  surface.  Overheating,  however,  de- 
prives it  of  carbon,  when  the  fractured  surface  pre- 
sents a  coarse,  granular  condition,  showing  that  it  is 
unfit  for  use  for  fine  cutting-instruments. 

Case-hardening  consists  in  conferring  the  hardness 
of  steel  upon  the  external  surface  of  iron  objects 
which  are  to  be  subjected  to  considerable  wear,  such 
as  gun-locks,  etc.,  and  is  accomplished  by  heating 
them  in  some  substance  rich  in  carbon  (such  as  bone- 
dust,  cyanid  of  potassium,  etc.),  and  afterward  chill- 
ing in  water.  The  body  of  the  piece  so  treated  retains 
the  toughness  of  iron. 

Malleable  iron  is  produced  by  a  process  the  reverse 
of  that  employed  in  case-hardening.  It  consists  in 
heating  the  object,  usually  made  of  cast  iron  (when 
great  softness  and  tenacity  are  required),  for  some 
hours  in  contact  with  oxid  of  iron  or  manganese,  by 
which  its  carbon  and  silicon  are  removed. 

A  steel  instrument  may  be  readily  distinguished 
from  iron  by  placing  a  drop  of  nitric  acid  upon  it,  a 
dark  stain  being  produced  upon  steel  by  the  separa- 
tion of  the  carbon. 


CHAPTER   XIV. 

MERCURY. 
Atomic  Weight,  200.     Symbol,  Hg  (Hydrargyrum). 

MERCURY  (quicksilver)  has  been  known  from  a 
very  early  period.  It  is  the  only  metal  which 
is  liquid  at  ordinary  temperatures,  but  it  be- 
comes solid  at  390  F.  below  zero.  It  is  frequently 
found  in  nature  in  the  metallic  state,  and  it  is  proba- 
ble that  the  first  supplies  of  the  metal  were  from  this 
source.  The  ancients  distinguished  between  such 
mercury  and  that  obtained  by  reduction  from  the  ores 
by  designating  the  native  metal  as  argentum  vivum 
and  the  reduced  as  hydrargyrum, — a  fact  which  has 
led  some  to  doubt  whether  they  were  believed  to  be 
the  same  metal. 

The  sources  of  mercury  are  cinnabar,  or  mercuric 
sulfid  (the  ordinary  ore  from  which  the  metal  is  ob- 
tained) ;  horn  quicksilver,  or  native  calomel  ;  native 
amalgam  of  silver  and  mercury,  sometimes  found 
trickling  from  crevices  in  the  ores.  It  is  also  occa- 
sionally found  in  globules  disseminated  through  the 
native  sulfid. 

Occurreiice.  —  It  is  found  in  considerable  quantities 
in  Almaden  in  Spain,  Idria  in  Austria,  and  has  been 
found  in  great  abundance  and  of  remarkable  purity 
in  California  and  Australia.  The  metal  is  extracted 
from  the  sulfid  at  Idria  by  roasting  the  ore  in  a  kiln, 
which  is  connected  with  an  extensive  series  of  con- 
densing-chambers   built  of  brick-work.      The   sulfur 

15  217 


2l8  DENTAL    METALLURGY. 

is  converted  by  the  air  in  the  kiln  into  sulfurous  acid 
gas,  while  the  mercury  passes  off  in  vapor,  and  is 
condensed  in  the  chambers.  The  greater  portion  of 
the  mercury  of  commerce  is  produced  at  the  Aus- 
trian mine  at  Idria,  whence  it  is  exported  in  bottles  of 
hammered  iron,  containing  seventy-five  pounds  each, 
in  a  state  often  nearly,  though  never  quite,  pure.  It 
is,  however,  frequently,  when  purchased  in  small 
quantities,  adulterated  with  tin  and  lead.  The  pres- 
ence of  debasing  metals  such  as  these  may  be  detected 
by  scattering  a  little  upon  a  clean  glass  plate,  when  it 
* '  tails' '  or  leaves  a  track  upon  the  glass .  Lead,  which 
is  its  most  frequent  impurity,  may  be  removed  by  ex- 
posing the  mercury  in  a  thin  layer  (in  a  broad,  shallow 
dish)  to  the  action  of  nitric  acid  diluted  with  two  parts 
of  water,  which  should  cover  its  surface  and  be  allowed 
to  remain  in  contact  with  it  for  several  days,  with  occa- 
sional stirring.  The  lead  is  thus  oxidized  by  the  acid, 
and  after  digestion  may  be  washed  away. 

Distillation  is  the  means  most  frequently  recom- 
mended for  the  purification  of  mercury,  but  it  is  now 
regarded  as  an  uncertain  one,  since,  if  zinc  or  bismuth 
be  present,  they  will  be  sure  to  distil  over  into  the  re- 
ceiver with  the  mercury. 

In  distilling  or  redistilling  mercury  a  strong  glass 
retort  should  be  used,  in  size  corresponding  with  the 
quantity  of  the  metal  to  be  operated  upon,*  and  filled 
one-third  with  mercury,  upon  the  surface  of  which  is 
placed  a  layer  of  clean  iron  filings.  The  retort  should 
be  imbedded  in  a  sand-bath,  with  the  neck  made  to 
incline  so  as  to  dip  below  the  surface  of  the  water, 

*  For  quantities  exceeding  two  or  three  pounds  iron  retorts  are  em- 
ployed. 


MERCURY.  219 

with  which  the  receiver  is  half  filled  ;  and,  in  order  to 
facilitate  condensation  of  the  metallic  vapor,  the  re- 
ceiver should  be  surrounded  by  cold  water.  Should 
there  be  a  film  of  oxid  upon  the  surface  of  the  distilled 
mercury,  a  small  quantity  of  hydrochloric  acid  will 
dissolve  it,  when  the  mercury  may  be  washed  and 
dried  at  a  moderate  heat.  Another  and  better  method 
is  to  substitute  coarsely-powdered  cinnabar  for  the 
iron  filings.  The  sulfur  liberated  from  the  former  con- 
verts foreign  metals  into  sulfids,  while  the  mercury 
present  is  liberated. 

Probably  the  most  certain  means  of  obtaining  mer- 
cury free  from  admixture  of  other  metals  is  to  operate 
upon  one  of  the  salts  of  mercury  ;  as,  for  instance,  the 
red  oxid,  the  bichlorid  (corrosive  sublimate),  or  pure 
vermilion.  These  are  readily  decomposed  by  heat, 
and  when  the  red  oxid  is  employed,  simple  distillation 
is  sufficient.  Should  the  metal  after  distillation  show 
a  film  of  oxid,  the  latter  may  be  easily  removed  with 
dilute  nitric  acid  slightly  warmed.  Where  vermilion 
or  the  chlorid  is  to  be  reduced,  the  distillation  will  be 
greatly  facilitated  by  the  addition  of  one  part  of  lime. 

It  is  important  that  the  mercury  employed  for 
dental  purposes  should  be  quite  pure,  and  doubtful 
specimens  may  be  effectually  freed  of  the  presence  of 
the  adulterating  metals  named  by  the  following  very 
simple  means  :  A  small  quantity  of  mercurous  nitrate 
is  dissolved  in  water,  and  into  this  the  impure  mercury 
is  placed.  The  salt  will  be  decomposed,  the  adulter- 
ating metals  oxidized,  and  they  will  be  replaced  by 
the  mercury  of  the  salt.  The  same  result  may  also 
be  attained  by  digesting  in  a  solution  consisting  of  one 
part  of  nitric  acid  and  eight  parts  of  water  for  several 


220  DENTAL     METALLURGY. 

hours  at  a  temperature  of  1300  or  1400  F.  The  mer- 
cury should  be  placed  in  a  flask  or  glass  dish  with  a 
broad,  shallow  bottom,  and  will  require  frequent 
shaking  or  stirring.  The  dilute  acid  has  little  or  no 
effect  upon  the  mercury,  while  it  readily  dissolves 
and  combines  with  the  impurities. 

Another  very  simple  means,  said  to  have  been  de- 
vised by  Dr.  Priestley,  has  frequently  been  employed 
in  purifying  mercury.  It  consists  in  placing  the  metal 
with  some  fine-ground  loaf-sugar  in  a  bottle,  which, 
after  being  securely  corked,  is  vigorously  shaken  for 
a  short  time  ;  it  is  then  opened  and  air  blown  in  by 
means  of  a  bellows  ;  again  stopped  and  shaken  as 
before,  and,  after  repeating  this  treatment  three  or 
four  times,  the  mercury  is  filtered  through  a  cone 
made  of  smooth  paper,  with  a  fine  pin-hole  at  its 
apex.  The  sugar,  to  which  the  oxids  of  foreign 
metals  adhere,  remains  behind. 

The  method  of  filtering  mercury  through  chamois- 
leather  is  to  most  dentists  a  familiar  means  of  exclud- 
ing a  superabundance  of  mercury  from  amalgams,  but 
is  not  to  be  relied  on  as  a  means  of  affording  pure 
mercury. 

Properties. — As  before  stated,  mercury  is  fluid 
above  a  temperature  of — 390  F.  Below  that  point, 
however,  it  solidifies,  and  may  be  hammered  or 
welded  like  other  metals.  During  solidification  it 
contracts  and  exhibits  octahedral  crystals.  When 
pure,  the  fluid  metal  is  exceedingly  lustrous.  It  boils 
at  about  66o°  F.  It  is  volatile  even  at  the  ordinary 
temperatures,  and  this  property  is  greatly  increased 
by  heat.  There  have  been  curious  illustrations  of  the 
disposition   to   assume   a   state  of  vapor  which  this 


MERCURY.  221 

metal  evinces. *  It  is  readily  soluble  in  strong  nitric 
acid,  but  is  dissolved  in  sulfuric  acid  only  by  the 
assistance  of  heat.  Hydrochloric  acid  has  no  effect 
upon  it 

When  shaken  in  air,  or  rubbed  with  grease,  turpen- 
tine, or  other  substances,  such  as  sugar,  chalk,  etc., 
mercury  loses  its  metallic  appearance,  and  is  converted 
into  a  gray  mass,  as  in  mercurial  ointment,  etc.  This 
was  formerly  regarded  as  an  oxidation  of  the  metal, 
and  the  belief  was  probably  favored  by  the  fact  that 
mercury,  in  a  state  of  fine  division,  becomes  active 
for  therapeutic  purposes.  The  microscope,  however, 
has  revealed  that  such  preparations  are  composed  of 
metallic  globules  of  about  j±q  of  a  line  in  diameter. 

Mercury  amalgamates  more  or  less  readily  with 
gold,  silver,  tin,  zinc,  lead,  bismuth,  cadmium,  potas- 
sium, etc.,  and  with  a  greater  degree  of  difficulty  with 
platinum,  palladium,  and  copper.  In  some  instances 
combination  between  mercury  and  other  metals  takes 
place  with  considerable  violence,  as  in  the  case  of 
potassium,  in  which  light  and  heat  are  developed. 
Many  of  the  amalgams  become  solid  and  crystalline, 
as  illustrated  in  dental  amalgams,  and  for  the  most 
part  they  may  be  looked  upon  as  definite  compounds. 
Indeed,  a  native  compound  of  mercury  and  silver  has 
been  found  crystallized  in  octahedra  and  other  forms 
of  the  regular  system.  The  liquid  amalgams  are 
probably  in  many  instances  solutions  of  definite  com- 
pounds in  an  excess  of  mercury  ;  if  these  are  pressed 
through    chamois-leather   the    mercury   is  excluded, 

*  Burnet  relates  an  instance  in  which  some  mercury  in  leather  bags, 
forming;  part  of  the  cargo  of  a  ship,  escaped  into  the  bilge  of  the  vessel. 
An  elastic  Huid  was  soon  evolved,  which  covered  every  metallic  article  on 
board,  while  the  crew,  to  a  man,  were  salivated. 


222  DENTAL   METALLURGY. 

carrying  with  it  but  a  small  portion  of  the  other 
metals,  while  a  solid  amalgam,  frequently  of  definite 
atomic  constitution,  remains  behind. 

Compounds. — Mercury  unites  with  oxygen  to  form 
mercuric  and  mercurous  oxids,  both  of  which  are 
highly  poisonous.  With  chlorin  it  forms  two  com- 
pounds, mercuric  chlorid  (HgCl2)  commonly  called 
corrosive  sublimate,  and  mercurous  chlorid  (Hg2Cl2), 
familiarly  known  as  calomel.  Mercuric  chlorid  is  a 
valuable  germicide,  and  as  such  is  extensively  used  by 
dentists  in  the  treattnent  of  devitalized  teeth.  With 
iodin,  mercury  forms  two  compounds,  mercuric  iodid 
(Hgl2),  and  mercurous  iodid  (Hg2I2).  The  former  is 
also  regarded  as  a  valuable  antiseptic  agent.  With 
sulfur,  however,  mercury  combines  to  form  sulfates 
and  sulfids.  Of  the  latter  we  have  mercuric  sulfid 
(HgS),  a  compound  of  great  interest  to  dentists  in 
consequence  of  its  extensive  use  as  a  coloring  pigment 
in  vulcanizable  rubbers  and  celluloid.  It  is  the  most 
common  ore  of  mercury,  and,  as  such,  is  termed 
cinnabar  ;  when  produced  artificially  it  is  known  as 
vermilion.  The  best  quality  of  the  latter  is  made  by 
the  Chinese.  Their  process  of  manufacturing,  for  a 
long  time  a  secret,  consists  in  stirring  a  mixture  of  one 
part  of  sulfur  and  seven  parts  of  mercury  in  an  iron 
pot ;  chemical  union  takes  place,  the  result  of  the 
combination  being  a  black  powder.  This  is  divided 
into  small  lots,  which  are  emptied  separately  into 
suitable  subliming  pots,  heated  to  redness.  When  a 
sufficient  quantity  has  been  placed  in  the  pots  they  are 
covered  up,  and  the  heat  is  continued  for  thirty-six 
hours,  with  occasional  stirring  by  means  of  an  iron  rod 
passed  through  the  lid.     Lastly,  the  pots  are  broken, 


MERCURY.  223 

and  the  vermilion  adhering  to  the  upper  portions  levi- 
gated and  dried.  It  may  also  be  formed  by  rubbing 
three  hundred  parts  of  mercury  with  one  hundred  and 
fourteen  parts  of  flowers  of  sulfur,  moistened  with 
a  solution  of  caustic  potash.  The  resulting  product, 
which  is  black,  is  then  digested  at  about  1200  F.  with 
seventy-five  parts  of  hydrate  of  potash  and  four  hun- 
dred of  water,  until  it  acquires  a  fine  red  color. 

Vermilion  may  be  adulterated  with  red  lead,  disulfid 
of  arsenic  (As,S,),  ferric  oxid,  brick-dust,  or  any 
cheaper  substance  of  a  similar  color  ;  and  the  discom- 
fort sometimes  caused  by  wearing  vulcanized  rubber 
artificial  dentures  may  be  in  part  due  to  the  presence 
of  such  deleterious  substances  as  the  arsenic  and  lead 
salts  referred  to. 

Vermilion  is  an  inert  mercurial  compound,*  and  is 
quite  insoluble  in  either  nitric,  sulfuric,  or  hydrochloric 
acid.  It  is  unaffected  by  water,  alcohol,  or  the  alkalies. 
Nitro-hydrochloric  acid  (aqua  regia),  however,  dis- 
solves and  converts  it  into  corrosive  sublimate  (HgCL), 
and  by  exposure  to  a  temperature  of  6oo°  F.  vermilion 
is  decomposed,  and  the  reduced  metal  may  be  col- 
lected in  globules  by  condensing  the  vapor. 

Pure  vermilion,  in  combination  with  rubber,  is  not 
likely  to  produce  deleterious  effects  when  worn  in  the 
mouth,  nor  is  it  probeible  that  this  compound  can  be 
decomposed  chemically  and  converted  into  a  poisonous 
salt  of  mercury  by  mere  contact  with  the  saliva.  The 
mechanical  dentist  will,  however,  do  well  to  avoid  the 
use  of  nitro-hydrochloric  acid  in  removing  tin  foil 
from  the  surface. 

*  In  the  less  civilized  countries  the  ancients  used  cinnabar  to  paint  their 
bodies,  without  any  bad  effects.     Makins,  p.  128. 


224  DENTAL   METALLURGY. 

Regarding  the  presence  of  free  mercury  in  rubbers 
before  or  after  vulcanizing,  Prof.  Austen  stated  that 
the  researches  of  Prof.  Johnston  with  the  microscope, 
and  of  Prof.  Mayr  by  chemical  analysis,  failed  to  dis- 
cover the  slightest  trace  in  samples  of  that  which  had 
been  used  by  him  for  several  years.  Prof.  Wildman 
observed  that  sulfur  sublimed  during  vulcanization, 
but  did  not  find  the  smallest  trace  of  free  mercury.* 
Prof.  Austen  failed  by  mechanical  force  to  press  out 
any  metallic  globules,  and  during  his  entire  experience 
with  indurated  rubber  as  a  base  for  artificial  dentures 
never,  even  with  the  microscope,  detected  the  slight- 
est particle  of  metallic  mercury  upon  the  surface  of 
any  finished  piece. 

If  it  is  true,  as  some  assert,  that  free  mercury  has 
occasionally  been  observed  in  rubber,  then  its  presence 
must  have  been  due  to  the  use  of  an  imperfect  quality 
of  vermilion  ;  but  that  the  latter,  when  pure,  is  ever 
reduced  during  the  process  of  vulcanizing,  or  by  wear- 
ing in  the  mouth,  is  not  at  all  probable. 

The  modified  condition  of  that  portion  of  the  sur- 
face of  the  mouth  in  contact  with  the  rubber  plate, 
often  accompanied  by  a  sensation  of  heat,  has  been 
attributed  to  an  electrical  action,  due  to  the  fact  that 
rubber,    "like   sealing-wax,   is    a    powerful  negative 

electric,  "f 

The  real  solution  of  the  question,  however,  will 
probably  be  found  in  the  following  conditions  :  ist. 
The  non-conducting  quality  of  the  substance.  2d. 
The  rough  condition  of  the  surface,  due  to  careless- 
ness or  want  of  skill  in  construction.     3d.  Want  of 

*  Harris's  Principles  and  Practice  of  Dentistry,  p.  682. 
f  Ibid.,  p.  681. 


MERCURY. 


^D 


care  on  the  part  of  the  wearer,  in  not  frequently  clean- 
ing the  piece  of  portions  of  food  and  the  secretions  of 
the  mouth,  which  are  likely  to  undergo  chemical  change 
by  long  confinement  in  contact  with  the  tissues,  and 
thus  become  irritants. 

The  author  has  frequently  noticed  this  inflamed  con- 
dition where  the  denture  was  of  gold  or  silver,  but 
always  in  cases  where  the  plate  was  seldom  removed 
or  cleansed.  It  is  true,  however,  that  the  trouble 
referred  to  is  more  common  in  rubber  or  celluloid 
work  ;  but  in  both  of  these  there  are  more  conditions 
favoring  such  a  result  than  are  found  in  a  metallic 
denture.  The  facts  that  the  symptoms  are  not  con- 
stant, and  that  by  far  the  greater  number  of  mouths 
in  which  rubber  or  celluloid  is  worn  are  not  in  the  least 
affected  by  it,  would  seem  to  confirm  this  view. 

Discriminatioyi. — The  presence  of  the  soluble  salts 
of  mercury  may  be  detected  by  Reinsch's  test, *  which 
consists  in  placing  a  clean  strip  of  copper  in  the  solu- 
tion ;  metallic  mercury  will  be  immediately  deposited 
upon  it,  giving  it  a  silvery-white  appearance.  The 
strip  of  copper  is  then  heated  in  a  glass  tube,  by 
which  the  mercury  is  sublimed,  and  may  be  detected 
in  the  form  of  minute  globules  adhering  to  the  sides 
of  the  tube  after  condensing. 

Insoluble  compounds  of  mercury  may  be  detected 
by  placing  a  small  portion  in  a  glass  tube  and  cover- 
ing with  a  layer  of  dry  sodium  carbonate,  and  then 
heating.  If  mercury  be  present,  it  will  separate  and 
condense  in  globules  in  the  cool  parts  of  the  tube. 
This  test  is  based  on  the  fact  that  all  mercurial  com- 
pounds are  decomposable  by  a  temperature  of  ignition. 

*See  works  on  chemistry. 


226  DENTAL   METALLURGY. 

Mercury  is  also  precipitated  from  its  soluble  com- 
binations by  a  solution  of  stannous  chlorid  used  in 
excess. 

Hydrogen  sulfid  and  ammonium  sulfid  produce  in 
solutions,  both  of  mercuric  and  mercurous  salts,  black 
precipitates  insoluble  in  ammonium  sulfid.  The  quan- 
tity of  the  reagent  should  be  sufficient  to  produce 
complete  decomposition  ;  otherwise,  a  white  precipi- 
tate will  be  formed,  consisting  of  mercuric  sulfid,  with 
the  original  salt.  An  excess  of  hydrogen  sulfid,  how- 
ever, instantly  turns  the  precipitate  black.  This  reac- 
tion is  regarded  as  characteristic  of  mercuric  salts. 

Caustic  potassa  or  soda,  when  added  to  solutions  of 
mercuric  salts,  produces  a  yellow  precipitate. 

Ammonia  or  ammonium  carbonate  produces  a  white 
precipitate  insoluble  in  excess. 

Potassium  or  sodium  carbonate  causes  a  red-brown' 
precipitate. 

Potassium  iodid  throws  down  a  bright  scarlet  pre- 
cipitate, soluble  in  excess  either  of  the  mercuric  salt 
or  of  the  alkaline  iodid. 


CHAPTER   XV. 

COPPER. 
Atomic  Weight,  63.4.    Symbol,  Cu  (Cuprum). 

COPPER  is  a  metal  with  which  mankind  has  been 
acquainted  from  the  most  remote  periods,  and 
probably  the  first  metallic  compound  employed 
was  copper  alloyed  with  tin  (bronze),  of  which  many 
relics  in  the  form  of  arms,  ornaments,  and  domestic 
implements,  evidently  belonging  to  an  early  period  in 
prehistoric  times,  are  still  to  be  found. *  It  is  proba- 
ble, however,  that  the  production  of  the  pure  metal  is 
an  operation  of  a  more  recent  date. 

Copper  ores  are  found  in  many  parts  of  America 
and  Europe.  In  some  parts  of  the  United  States  the 
native  metal  is  found  in  immense  masses  many  hun- 
dred pounds  in  weight,  sometimes  slightly  intermixed 
with  silver.  Nothing  is  certainly  known  of  the  origin 
of  these,  but  they  are  supposed  to  have  been  formed 
from  the  cupric  sulfid,  which,  by  exposure  to  air  and 
moisture,  was  converted  into  sulfate,  and  then,  by 
electro-chemical  agency,  reduced  to  the  metallic  state. 

There  are  several  ores  which  yield  copper.  The 
one  most  commonly  employed,  however,  is  copper 
pyrites,  a  combination  of  sulfid  of  copper  and  iron. 
The  blue  and  green  carbonates,  known  respectively  as 
azurite  and  malachite,  are  beautiful  minerals,  exten- 
sively used  in  Russia  and  Bohemia  in  the  manufacture 

*  The  epoch  marked  by  the  use  of  bronze  is  known  in  archaeological 
chronology  as  the  Bronze  Age. 

227 


228  DENTAL    METALLURGY. 

of  ornamental  objects.  They  contain  upwards  of  fifty 
per  cent,  of  copper. 

The  process  of  obtaining  copper  from  an  ore,  such 
as  copper  pyrites,  may  be  thus  briefly  described  :  The 
ore  is  heated  in  a  reverberatory  furnace  for  the  pur- 
pose of  converting  the  iron  sulfids  into  oxid.  The 
copper,  which  remains  unaltered,  is  then  heated  with 
a  siliceous  sand,  which  combines  with  the  iron  oxid  to 
form  a  slag,  and  separates  from  the  heavier  (copper) 
compound.  By  repeating  this  process  the  iron  is 
finally  got  rid  of,  when  the  copper  sulfid  begins  to 
decompose  in  the  flame-furnace,  parting  with  its  sul- 
fur and  absorbing  oxygen.  The  resulting  oxid  is, 
however,  reduced  by  the  aid  of  carbonaceous  matter 
and  a  high  degree  of  heat. 

Properties. — Pure  copper  may  be  obtained  by 
decomposing  a  solution  of  pure  sulfate  of  copper  in 
the  galvanic  current.  If  the  negative  wire  be  attached 
to  a  copper  plate  immersed  in  the  solution,  the  pure 
metal  will  be  deposited  on  it,  and  may  be  readily 
stripped  off. 

The  chief  value  of  copper  in  the  useful  arts  is  due 
to  its  great  malleability,  in  which  quality  it  is  only 
exceeded  by  gold  and  silver.  It  fuses  at  about  20000 
F.  It  expands  in  solidifying,  and  absorbs  oxygen 
very  much  in  the  same  manner  as  silver  does  under 
similar  conditions.  In  tenacity  copper  ranks  next  to 
iron,  as  a  copper  wire  of  one-tenth  of  an  inch  in  diam- 
eter will  support  about  385  pounds.  Its  power  of  con- 
ducting electricity  is  nearly  equal  to  that  of  silver, 
while  in  the  transmission  of  heat  it  is  surpassed  only 
by  silver  and  gold.  It  is  readily  soluble  in  nitric  acid, 
but  in  sulfuric  acid  only  with   the  assistance  of  heat. 


COPPER.  229 

Hydrochloric  acid  attacks  it  slowly,  and  in  vacuo  is 
inactive.  The  specific  gravity  of  copper  is  8.93.  For 
the  compounds  of  copper  with  the  non-metallic  ele- 
ments the  student  is  directed  to  Fownes's,  Bloxam's, 
and  other  works  on  chemistry. 

Amalgams. — Copper  does  not  readily  unite  with 
mercury  without  the  assistance  of  heat.  There  is, 
however,  an  amalgam  of  pure  copper  and  mercury  ex- 
tensively used  in  Europe  under  the  name  of  Sullivan's 
amalgam.  Its  preparation  is  as  follows  :  Pure  copper 
in  a  finely-divided  state  is  obtained  by  boiling  a  con- 
centrated solution  of  cupric  sulfate  with  distilled  zinc 
until  the  blue  color  of  the  salt  disappears,  when  the 
zinc  should  be  removed.  The  copper,  which  will  be 
found  in  a  pulverulent  mass  at  the  bottom  of  the  ves- 
sel, should  be  washed  with  dilute  sulfuric  acid,  sub- 
sequently in  hot  distilled  water,  and  dried.  It  is  then 
moistened  with  a  solution  of  nitrate  of  mercury,  by 
which  means  the  copper  becomes  completely  coated 
with  mercury.  The  mercury  is  then  added  to  it  to  the 
extent  of  twice  the  weight  of  copper  (3  of  copper  to 
6  of  mercury).  It  is  then  rolled  into  small,  lozenge- 
shaped  pieces,  which  become  quite  hard,  and  are 
supplied  to  the  profession  in  bottles  containing  an 
ounce  or  more.  This  amalgam  possesses  the  property 
of  softening  with  heat  and  again  hardening,  and  when 
employed  as  a  filling-material  one  of  the  lozenge- 
shaped  pieces  is  placed  in  a  small  iron  spoon  made 
and  sold  for  the  purpose,  and  heated  over  the  flame 
of  a  spirit-lamp  until  small  globules  of  mercury  are 
driven  to  the  surface,  when  it  is  placed  in  a  small  glass 
or  porcelain  mortar  and  rubbed  into  a  smooth  paste. 
Some  recommend  washing  with  a  weak  solution  of 


230  DENTAL   METALLURGY. 

sulfuric  acid,  or  soap  and  water,  and  lastly  with  clean 
water  alone,  to  remove  the  last  traces  of  either  acid  or 
soap,  and  finally  squeezing  through  chamois-leather 
to  exclude  surplus  of  mercury,  when  it  is  ready  to  be 
introduced  into  the  cavity.  It  requires  several  hours 
to  harden.  Mr.  Fletcher  says  of  this  amalgam  that 
"it  is  an  absolutely  permanent  filling,  as  the  copper 
salts  permeate  and  perfectly  preserve  the  tooth."  It 
is  said  to  be  quite  insoluble  in  the  mouth.  It,  how- 
ever, becomes  intensely  black,  and  imparts  a  most  ob- 
jectionable stain  to  the  teeth. 

According  to  Watt's  "Chemical  Dictionary,"  the 
specific  gravity  of  pure  copper  amalgams  is  the  same 
after  hardening  as  before,  ' '  hence  the  presence  of 
copper  in  amalgam  alloys  lessens  their  contractibility. ' ' 

It  is  said*  that  the  tendency  of  copper  amalgam  to 
discolor  may  be  lessened  by  careful  attention  to  its 
preparation.  The  older  way  of  preparing  it  was  by 
precipitating  copper  from  a  solution  of  cupric  sulfate, 
with  mercury  at  the  bottom  of  the  vessel  that  con- 
tained it,  by  stirring  the  fluid  with  a  bar  of  zinc  ;  but  a 
better  way,  and  one  now  employed,  is  to  substitute  a 
clean  iron  bar  for  the  zinc,  and  leave  it  from  twelve  to 
twenty-four  hours  in  a  jar  containing  the  solution. 
The  iron  bar  becomes  covered  with  a  dull  red  floccu- 
lent  precipitate  of  copper.  When  a  sufficient  quantity 
of  the  precipitate  is  formed  it  is  collected  into  another 
jar,  and  well  washed  with  a  stream  of  cold  water  until 
it  becomes  quite  clean.  It  is  then  ground  in  a  mortar 
until  it  begins  to  amalgamate,  the  amalgamation  being 
hastened  by  hot  water  slightly  acidulated  with  sulfuric 
acid,  which  will  also  remove  traces  of  iron.     It  should 

*  Mr.  E.  P.  Collett,  British  Journal  of  Dental  Science,  April  15,  1890. 


COPPER.  23I 

next  be  washed  in  liquor  ammonia  to  neutralize  traces 
of  acid,  and  must  then  be  thoroughly  triturated  in  a 
mortar  until  thorough  amalgamation  has  been  effected. 
The  amalgam  should  be  rolled  into  small  pellets,  and 
allowed  to  set  for  twenty-four  hours  before  using. 

The  expectations  of  valuable  therapeutic  qualities 
in  copper  amalgams  have  not  been  realized.  Drs.  C. 
D.  Cook  and  W.  St.  G.  Elliott,  of  London,  found 
by  experiment  that  copper  amalgams  shrunk  more 
than  simple  alloys  of  tin  and  silver,  and  the  opinion 
seems  to  be  gaining  ground  among  those  who  have 
recently  somewhat  eagerly  adopted  it  as  a  filling- 
material,  that  it  is  a  very  uncertain  agent,  and  that 
"whatever  antiseptic  influence  it  has,  it  does  not 
prevent  decay  from  beginning  and  progressing  directly 
in  contact  with  it"* 

Alloys. — Copper  unites  readily  with  all  other  metals, 
and  many  of  the  resulting  alloys  are  of  great  value  in 
the  industrial  arts, — of  even  more  value  than  the  pure 
metal.  It  is  added  to  silver  for  the  purpose  of  con- 
ferring sufficient  hardness  upon  the  latter  to  enable  it, 
in  the  form  of  coin  or  plate,  to  withstand  the  attrition 
to  which  such  articles  are  exposed.  The  formation 
of  a  perfectly  uniform  alloy  of  silver  and  copper  is  a 
process  attended  with  some  uncertainty,  owing  to  a 
tendency  on  the  part  of  the  copper  to  separate  and 
pass  off  toward  the  edges  as  the  ingot  solidifies.  Thus, 
in  silver  coins  one  portion  of  the  piece  will  frequently 
be  found  to  contain  more  copper  than  another. 

The  decimal   proportions  of  copper  and  silver  in 

*  Dr.  Howe,  in  discussions  of  J.  Allen  Osmun's  paper  entitled  "  Some 
Observations  on  the  Use  of  Copper  Amalgams,"  read  before  the  New  York 
Odontological  Society,  published  in  the  International  Dental  Journal  for 
July,  1892. 


900, 

'       100. 

925, 

75. 

947,       ' 

53- 

811, 

'        189. 

283,       ' 

717. 

232  DENTAL     METALLURGY. 

standard  silver  (coin)  of  several  different  nationalities 
are  as  follows  : 

Of  the  United  States         .        .     silver  900,  copper  100. 
"  France  .... 

"  England        .... 
"  Indian  rupees 
"  Germany — Prussian  thalers 
"  Prussian  silver  groschen 

The  properties  conferred  upon  gold  by  the  addition 
of  copper  are  similar  to  those  imparted  to  silver. 
These  have  already  been  alluded  to  on  page  181.  The 
decimal  proportions  in  the  gold  coins  of  the  United 
States,  France,  and  Holland  are  :  gold,  900  ;  copper, 
100  ;  while  English  coins  are  composed  of  916.6  of 
gold  and  83.3  of  copper.  In  coin-gold  malleability  is 
not  greatly  interfered  with .  Gold  may,  however,  be 
rendered  brittle  by  large  proportions  of  copper,  or 
when  the  latter  is  impure. 

Copper  and  platinum  form  an  alloy,  when  the  pro- 
portions are  equal,  of  nearly  the  same  specific  gravity 
and  color  as  gold.  Copper  also  unites  with  palladium 
to  form  a  light,  brassy  alloy.  By  admixture  with  lead 
or  bismuth  copper  is  rendered  quite  brittle.  The 
principal  alloys  in  which  it  forms  a  leading  ingredient 
are  brass,  bronze,  and  German-silver. 

Aluminum  bronze  is  formed  of  pure  copper  alloyed 
with  from  2.5  to  10  per  cent,  of  aluminum.  It  is 
quite  malleable,  and  has  a  fine,  rich,  golden  color. 
Phosphor-bronze  is  copper  combined  with  from  three 
to  fifteen  per  cent,  of  tin  and  from  one-quarter  to  two 
and  a  half  per  cent,  of  phosphorus.  Other  metals, 
such  as  silver,  nickel,  cobalt,  antimony,  and  bismuth, 
frequently  enter  into  the  composition  of  bronzes. 


COPPER.  233 

Copper  in  small  quantities  (from  5  to  7  per  cent.) 
is  said  by  Mr.  Fletcher  to  confer  upon  amalgams  the 
quick-setting  property  obtained  by  the  addition  of 
platinum.  It  is,  however,  considered  inferior  to  plati- 
num as  a  constituent  in  dental  alloys  ;  but  in  the 
absence  of  platinum,  amalgams  are  improved  by  the 
addition  of  a  small  proportion  of  copper. 

It  is  stated*  that  an  alloy  of  tin  10,  silver  8,  gold  1, 
copper  1,  has  been  extensively  used  (in  England  prob- 
ably) under  the  names  of  gold  amalgam  and  platinum 
amalgam. 

Hydrogen  sulfid  (H2S)  and  ammonium  sulfid,  when 
added  to  a  copper  solution,  afford  a  brownish-black 
cupric  sulfid. 

Caustic  potash  throws  down  a  pale-blue  precipitate 
of  cupric  hydrate,  which  changes  to  a  blackish- brown 
anhydrous  oxid  on  boiling.  Ammonia  also  gives  a 
blue  precipitate,  soluble  in  excess,  affording  a  deep 
purplish  blue  solution. 

Potassium  ferrocyanid  gives  a  red-brown  precipitate 
of  cupric  ferrocyanid.  It  may  also  be  detected  in 
very  weak  solutions  by  placing  a  drop  on  a  slip  of 
clean  platinum  foil.  A  point  of  zinc  is  then  dipped  in 
so  as  to  touch  the  foil,  and  instantly  a  spot  of  reduced 
copper  appears. 

A  green  line  is  imparted  to  the  oxidizing  flame  of 
the  blow-pipe  when  a  copper  salt  is  heated  in  it.  It 
also  communicates  a  green  tint  to  borax  when  heated 
with  it. 

There  are  several  methods  which  may  be  advan- 
tageously employed  for  the  estimation  of  copper. 
The  operations  of  the  dentist,  however,   are  chiefly 

♦Fletcher. 
16 


234  DEXTAL    METALLURGY. 

confined  to  the  examination  of  amalgam  alloys.  The 
alloy  should  first  be  acted  upon  by  nitric  acid  ;  silver. 
if  present,  may  then  be  recovered  in  the  form  of 
chlorid  ;  after  which  the  copper  may  be  precipitated 
from  the  remaining  solution  either  as  oxid,  sulfid.  or 
in  the  metallic  state.  When  attempting  the  estima- 
tion of  an  alloy,  a  qualitative  examination  should  first 
be  made  (page  66),  and  if  the  solution  to  be  examined 
is  found  to  contain  no  other  metal  whose  oxid  is  thrown 
down  bv  caustic  potassa,  an  excess  of  that  agent  is  to 
be  added.  In  the  resulting  precipitate,  when  boiled, 
washed,  dried,  and  weighed,  every  one  hundred  parts 
mav  be  estimated  as  containing  79.85  per  cent,  of 
metallic  copper. 

When  hvdrogen  sulfid  or  ammonium  sulfid  is 
emploved  as  the  reagent,  the  resulting  cupric  sulfid  is 
usually  oxidized  by  nitric  acid,  and  again  precipitated 
by  potassa,  so  as  to  estimate  as  oxid. 

The  estimation  as  metallic  copper  is  accomplished 
as  follows  :  Place  in  the  solution  contained  in  a  plat- 
inum dish  a  piece  of  zinc,  adding  also  a  little  hydro- 
chloric acid.  The  electrolyzing  action  instantly  com- 
mences, and  continues  until  the  solution  is  colorless 
and  the  zinc  completely  dissolved.  The  finely-divided 
metallic  copper  will  be  found  at  the  bottom  of  the 
vessel.     This  is  to  be  well  washed,  dried,  and  weighed. 


CHAPTER   XVI. 

ZINC. 
Atomic  Weight,  65.2.    Symbol,  Zn. 

THE  ancients  were  undoubtedly  acquainted  with  an 
ore  (probably  cadmia*)  which  they  employed 
with  copper  to  form  brass.  Many  objects  of 
ancient  manufacture,  analyzed  at  different  times,  have 
been  found  to  contain  zinc.f  The  extraction  of  the 
metal  itself,  however,  is  probably  a  modern  discovery. 

Metallic  zinc  is  never  met  with  in  nature.  The 
principal  ores  are  the  red  oxid — the  sulfid  of  zinc 
(blende)  and  the  native  carbonate  (calamine).  The 
latter  is  the  most  valuable  of  the  zinc  ores,  and  is 
preferred  for  the  extraction  of  the  metal.  It  is  first 
roasted  to  expel  water  and  carbonic  acid  ;  then  mixed 
with  fragments  of  coke  or  charcoal,  and  distilled  at  a 
full  red  heat  in  an  earthen  retort.  Carbon  monoxid 
escapes,  while  the  reduced  metal  volatilizes  and  is 
condensed  by  suitable  means. 

Properties. — Zinc  is  a  brittle,  crystalline  metal,  with 
a  density  varying  from  6.8  to  7.2.  Until  about  the 
commencement  of  the  present  century  the  valuable 
property  possessed  by  this  metal,  of  becoming  quite 
malleable  between  2480  and  3020  F. ,  was  not  known  ; 
hence,  prior  to  that  discovery  it  was  but  little  used  in 
the  industrial  arts.     Between  these  degrees  of  heat  it 

*  An  ore  used  by  the  ancients,  containing  cadmium  and  zinc, 
f  Phillips    made   a   number   of  analyses   of  such  objects,  all  of  which 
showed  the  presence  of  zinc. 

235 


236  DENTAL     METALLURGY. 

may  be  rolled  or  hammered  without  the  least  danger 
of  fracture.  Sheet  zinc  of  commerce  is  manufactured 
by  this  means,  and  it  retains  its  malleability  when  cold. 
Zinc  fuses  at  7730  F.  (below  red  heat).  At  a  bright 
red  heat  it  boils  and  volatilizes,  and  if  heated  in  air 
combustion  takes  place,  during  which  it  unites  with 
the  atmospheric  oxygen  with  brilliant  incandescence. 
At  4100  F.  zinc  is  so  brittle  that  it  may  be  powdered 
in  a  mortar. 

Alloys. — With  mercury  zinc  forms  an  exceedingly 
brittle  amalgam.  The  two  combine  in  the  cold  state, 
but  union  is  greatly  facilitated  by  heating.  Zinc  is 
occasionally  employed  as  a  constituent  in  dental  alloys. . 
An  amalgam  has  been  suggested,  the  proportions 
of  which  are  "  approximately"  given  as,  tin,  50  odd  ; 
silver,  30  ;  gold,  5  to  7  ;  zinc,  2  to  4;  and  recent 
experiments  with  it  have  proved  so  satisfactory  that  it 
has  to  a  certain  extent  taken  the  place  of  platinum  in 
dental  amalgams. 

Added  to  silver,  in  the  proportion  of  2  of  zinc  to  1 
of  silver,  a  nearly  white,  malleable  alloy  results. 

The  color  of  gold  is  heightened  by  the  addition  of 
zinc,  while  its  malleability  is  greatly  impaired.  Makins 
states  that  gold  rendered  standard  by  zinc  is  a  green- 
ish-yellow, brittle  alloy,  with  a  specific  gravity  above 
the  mean. 

Combination  between  zinc  and  platinum  or  palladium 
may  be  effected  at  a  comparatively  low  temperature, 
and  it  is  accompanied  by  evolution  of  light  and  heat. 
It  is  stated  that  an  alloy  of  16  parts  of  copper,  7  of 
platinum,  and  1  of  zinc  closely  resembles  16-carat 
gold,  is  quite  malleable,  does  not  tarnish  in  air,  and  is 
capable  of  resisting  cold  nitric  acid. 


ZINC.  237 

Zinc  and  lead  mix  with  each  other  to  a  very  limited 
extent.  If  equal  parts  of  the  two  metals  are  melted 
together  and  allowed  to  cool,  they  will  be  found  to 
have  separated  into  two  layers,  the  upper,  and  conse- 
quently the  lighter  one,  zinc,  retaining  1.2  percent, 
of  the  lead,  while  the  lower  layer  consists  of  lead 
alloyed  with  1.6  per  cent,  of  zinc.  The  necessity  of 
carefully  keeping  these  two  metals  separate  in  all  mold- 
ing operations  in  the  dental  laboratory  will  readily  be 
appreciated,  as  a  failure  to  observe  precaution  in  this 
direction  will  be  followed  by  vexatious  consequences. 
If  by  accident  lead  becomes  mixed  with  the  zinc  used 
for  dies,  the  lead,  by  its  greater  specific  gravity,  will 
settle  to  the  bottom  and  fill  up  the  deeper  portions  of 
the  sand  matrix  representing  the  alveolar  ridge,  the 
most  prominent  part  of  the  die.  This  may  not  be 
discovered  until  an  attempt  to  swage  is  made,  when  the 
die  will  be  found  to  be  totally  unfit  for  the  purpose. 
In  such  cases  the  mixed  metal  should  be  discarded 
and  new  zinc  substituted. 

Zinc  and  tin  unite  in  all  proportions  without  diffi- 
culty. Alloys  of  zinc  and  tin  are  frequently  employed 
in  casting  dies  for  swaging  plates.  Richardson*  gives 
a  formula  for  an  alloy  consisting  of  zinc  4  parts,  tin  1 
part ;  which,  he  states,  fuses  at  a  lower  temperature, 
contracts  less  in  cooling,  and  has  less  surface-hardness 
than  zinc.  Fletcher,  however,  states  that  all  alloys  of 
zinc  and  tin  are  superior  to  zinc  alone  for  dies.  The 
impression  from  the  sand  he  believes  to  be  much  finer, 
and  the  shrinkage  in  cooling  greatly  reduced.  Zinc  2, 
tin  1,  is  given  as  the  best  proportion,  t     Makins  states 

*  Mechanical  Dentistry. 

t  Practical  Dental  Metallurgy,  p.  69. 


238  DENTAL   METALLURGY. 

that  zinc  and  tin,  when  combined  in  equal  proportions, 
form  a  white,  hard  alloy,  not  very  malleable  or  ductile, 
which  is  capable  of  being  worked  as  readily  as  brass. 

Zinc  and  copper  unite  in  various  proportions  to 
form  many  different  grades  of  brass,  known  respec- 
tively as  pinchbeck,  Manheim-gold,  similor,  Bath- 
metal,  Prince  Rupert's  metal,  Muntz'ssterro,  Gedge's 
and  Aich's  metals.  German  silver  and  the  Chinese 
alloys  known  as  pacfong  and  tutenag  are  also  alloys 
of  zinc  and  copper,  with  the  addition  of  nickel. 

Dies  and  Counter- Dies. — Zinc  is  the  metal  most 
commonly  employed  in  the  formation  of  dies  for 
swaging  plates,  and  is  superior  to  any  of  its  alloys.* 
Another  important  application  of  zinc  is  in  the 
formation  of  counter-dies.  The  die  is  placed  in  the 
iron  ring  when  a  Bailey  flask  is  employed,  or  in- 
vested in  the  molding-sand  and  then  surrounded  by 
a  suitable  iron  ring  in  the  old-fashioned  way.  The 
zinc  is  then  heated  and  poured  in  upon  the  zinc  die 
just  at  the  moment  of  complete  fusion.  Should 
the  metal  be  accidentally  allowed  to  remain  on  the 
fire  too  long,  and  thus  reach  a  higher  temperature 
than  is  necessary,  it  should  not  be  poured  until  it 
begins  to  solidify  at  the  edges.  The  belief  seems  to 
be  pretty  general  that  melted  zinc  cannot  be  poured 
upon  a  zinc  die  without  causing  cohesion, f  but  if  the 
necessary  precaution  regarding  the  proper  tempera- 
ture at  which  the  metal  is  poured  is  observed,  it  is 

*  The  author  has  not  found  the  alloys  of  zinc  and  tin  to  be,  in  any  respect, 
superior  to  zinc  alone  for  dies. 

t  If  the  melted  metal  be  poured,  at  a  temperature  of  8oo°  F.,  upon  a  die 
having  a  temperature  of  70  F.,  the  fused  zinc,  by  contact  with  the  iron  ring 
and  by  radiation,  will  lose  heat  enough  to  cause  its  temperature  to  fall  far 
below  the  fusing-point,  and  it  will  probably  not  impart  to  the  die  more 
than  4000  F. 


ZINC.  239 

impossible  for  union  to  take  place,  and  when  cool 
the  die  and  counter-die  will  separate  quite  as  readily 
as  though  the  latter  was  of  lead.  It  seems  strange 
that  this  valuable  expedient  for  the  dental  laboratory 
has  not  found  a  place  in  the  text-books  on  mechanical 
dentistry.  It  frequently  occurs  that  the  zinc  die  and 
lead  counter-die  are  totally  inadequate  to  bring  a  plate 
(particularly  if  the  latter  is  of  platinum-gold  or  irid- 
ium-platinum)  into  perfect  adaptation  to  all  parts  of 
a  model,  especially  where  the  palatal  arch  is  very 
deep  and  the  rugae  are  prominent. 

The  zinc  counter- die  is  also  of  especial  service  in 
partial  cases  where  a  number  of  teeth  remain.  These 
are  cut  off  from  the  plaster  model  previous  to  mold- 
ing within  one-sixteenth  of  an  inch  of  the  margin  of 
the  gum,  so  that  a  sufficiently  distinct  impress  may 
be  made  in  the  plate  to  serve  as  a  guide  in  filing  the 
latter  to  fit  around  the  natural  teeth. 

Where  the  swaging  is  likely  to  be  attended  with 
difficulty,  at  least  three  sets  of  dies  and  counter-dies 
should  be  made.  The  most  imperfect  of  these  should 
be  furnished  with  a  lead  counter-die,  and  used  as  a 
preliminary  die  upon  which  to  start  the  plate.  The 
next  in  quality  may  be  used  with  the  zinc  counter-die, 
and  the  nearest  perfect  of  the  three,  with  a  lead  coun- 
ter, reserved  as  a  finishing-die.  When  the  plate,  by 
means  of  the  horn  or  wooden  mallet  and  some  prelimi- 
nary swaging  with  a  light  hammer,  has  been  made  to 
assume  somewhat  the  form  of  the  die,  and  has  been 
carefully  carried  past  the  stage  when  pleating  or 
wrinkling  of  the  plate  is  likely  to  occur,  it  should  be 
trimmed  to  the  proper  dimensions,  annealed,  and 
placed  between  the  die  and  zinc  counter-die,  and  at 


240  DENTAL     METALLURGY. 

first  gently  tapped  with  a  hammer  until  the  die  passes 
well  into  the  counter- die,  when  one  or  two  sharp 
blows  with  a  heavy  hammer,  either  upon  the  die  or  its 
counter,  will  carry  the  plate  into  perfect  adaptation 
to  all  parts  of  the  former.  Some  slight  compression, 
however,  of  the  prominent  points  of  the  die  is  likely 
to  occur  in  the  use  of  the  zinc  counter,  so  that  it  will 
be  necessary  to  anneal  and  give  the  plate  two  or  three 
sharp  blows  between  the  finishing-die  and  its  lead 
counter- die  ;  after  which  it  will  be  found  to  perfectly 
fit  the  mouth,  without  any  attempt  to  compensate 
for  contraction  of  the  zinc*  It  will  be  seen  that  the 
zinc  counter-die  is  not  intended  to  supersede,  but  is 
merely  used  as  an  adjunct  to,  the  lead  counter,  and 
there  is  probably  no  better  means  of  carrying  the 
plate  to  the  deep  parts  of  the  model,  and  of  obtain- 
ing a  sharp,  well-defined  impress  of  the  rugae  and 
prominent  parts  of  the  model. 

Zinc  will,  under  favorable  conditions,  unite  with 
iron,  and  it  frequently  attacks  the  cast-iron  ladle  in 
which  it  is  melted,  and  may  penetrate  the  side  and 
escape  into  the  fire.  Accidents  of  this  kind,  how- 
ever, may  be  avoided  by  coating  the  inside  of  the 
ladle  with  whiting. 

Compounds  of  Zi?ic. — The  oxid  and  the  chlorid 
are  the  compounds  of  this  metal  most  frequently 
employed  by  dentists.  The  first  forms  the  chief 
ingredient  in  the  plastic  filling- materials  known  as 
oxychlorids  and  oxyphosphates.  Zinc  oxid  is  a 
white  powder,  the  product  of  the  combustion  of  the 
metal.     It  turns  yellow  on  heating,   but  resumes  its 

*The  subject  of  shrinkage  of  zinc  when  used  for  dies  in  forming  metallic 
plates  has  been  fully  referred  to  on  page  23. 


ZINX.  24I 

pure  white  color  on  cooling.  Chlorid  of  zinc,  pre- 
pared by  acting  upon  the  metal  with  hydrochloric  acid 
or  by  heating  metallic  zinc  in  chlorin,  is  a  fusible,  deli- 
quescent substance,  quite  soluble  in  water  and  alcohol. 

Oxychlorid  of  zinc,  the  well-known  filling-mate- 
rial, consists  of  a  powder  and  a  fluid.  The  first  is 
prepared  by  various  formulas.  One  in  common  use 
is  as  follows  :  Grind  together  in  a  mortar  borax  2 
grains,  fine  silex  1  grain,  oxid  of  zinc  30  grains. 
When  thoroughly  mixed  these  are  placed  together  in 
a  small  crucible  and  heated  to  bright  redness.  This 
is  called  the  frit,  and  when  cool  requires  grinding  to 
again  reduce  it  to  a  pulverulent  state.  It  is  then 
thoroughly  mixed  with  three  times  its  weight  of  cal- 
cined oxid  of  zinc.  The  fluid  usually  employed  with 
the  powder  consists  of  chlorid  of  zinc  diluted  with 
water  in  the  following  proportions  :  Deliquesced 
chlorid  of  zinc,  1  ounce  ;  water,  5  or  6  drams.  The 
oxyphosphate  powders  are  similar  mixtures.*  The 
fluid,  however,  is  prepared  by  dissolving  in  pure  water 
some  glacial  phosphoric  acid,  and  then  evaporating 
until  the  solution  attains  the  consistence  of  glycerin. 

The  presence  of  zinc  in  solution  is  distinguished  by 
the  following  reactions  :  A  white  precipitate  soluble 
in  excess  of  the  alkali  is  obtained  by  the  addition  of 
caustic  potash,  soda,  or  ammonia,  and  zinc  is  distin- 
guished from  all  other  metals  by  ammonium  sulfid, 
which  precipitates  white  sulfid  of  zinc,  insoluble  in 
caustic  alkalies. 

*  Oxid  of  zinc,  200  parts,  silex  8,  borax  4,  ground-glass  5,  levigated  under 
water  to  insure  complete  admixture,  then  dried  by  evaporation,  calcined 
at  a  white  heat,  and  pulverized,  has  been  found  to  be  equal  in  durability 
and  working  qualities  to  any  of  the  numerous  oxyphosphates  now  in  the 
market. 


CHAPTER    XVII. 

CADMIUM. 
Atomic  Weight,  112.     Symbol,  Cd. 

OADMIUM,  a  metal  closely  allied  to  zinc,  was  dis- 
\y  covered  by  Stromeyer  and  Hermann  in  18 1 7.  It 
does  not  occur  in  the  metallic  state,  and  there  is 
only  one  definite  mineral  known  which  contains  it  in 
quantity,  namely,  the  sulfid,  or  greenockite,  which 
is  found  in  Renfrewshire,  Scotland.  This  contains 
77.7  per  cent,  of  cadmium  and  22.3  per  cent,  of  sulfur. 

The  production  of  cadmium  is  confined  to  a  very 
few  localities  in  Belgium  and  England.  It  occurs  in 
zinc  blende  to  the  extent  of  about  0.2  per  cent.  In 
the  calcination  of  the  blende  the  cadmium,  volatilizing 
at  a  lower  temperature  than  the  zinc,  passes  off  before 
the  latter  assumes  the  form  of  vapor.  The  oxid  of 
cadmium  is  collected  in  condensing-tubes,  and  is  sub- 
sequently reduced  to  the  metallic  state  by  heating  with 
carbon. 

Cadmium  is  a  white  metal  with  a  slight  bluish  tinge. 
It  is  somewhat  lighter  in  color  than  zinc  or  lead.  It 
is  susceptible  of  a  high  polish.  It  has  the  fibrous 
fracture  characteristic  of  soft,  tough  metals.  It  differs 
from  zinc  in  its  crystalline  form,  that  of  cadmium  being 
the  octahedral,  while  that  of  zinc  is  rhombohedral. 
It  is  harder  than  tin,  but  not  so  hard  as  zinc,  and  is 
sufficiently  malleable  to  admit  of  rolling  into  thin 
sheets.  Its  specific  gravity  after  fusion  is  8.6.  Its 
electric  conductivity  is  22.10, — somewhat  lower  than 
242 


CADMIUM.  243 

that  of  zinc.  It  melts  at  a  temperature  below  redness 
(3i50to3200C.). 

Cadmium  has  been  used  in  the  formation  of  dental 
alloys,  but  its  employment  as  a  constituent  in  amal- 
gams is  now  so  generally  condemned  that  it  is  seldom 
used  for  that  purpose. 

Probably  the  best  test  for  cadmium  is  the  color 
afforded  when  it  is  volatilized  and  oxidized  under  the 
blow- pipe  flame.  This  is  a  reddish-brown,  zinc  under 
the  same  conditions  giving  a  deposit  which  is  a  bright 
yellow  while  hot,  becoming  white  on  cooling.  It  may 
also  be  detected  by  precipitation  from  an  acid  solution 
as  a  yellow  sulfid,  and  may  in  this  way  be  distinguished 
from  zinc,  as  zinc  sulfid  does  not  separate  except  from 
neutral  or  alkaline  solutions. 

In  quantitative  analysis  cadmium  is  always  estimated 
as  oxid,  being  separated  from  solution  as  carbonate 
by  precipitation  with  carbonate  of  sodium,  which  is 
converted  into  oxid  by  calcination.  It  may  also  be 
separated  from  its  solution  in  acids  by  means  of  zinc, 
which  precipitates  it  in  a  dendritic  form,  like  the  well- 
known  lead  tree. 


CHAPTER   XVIII. 

ALUMINUM. 
Atomic  Weight,  27.4.    Symbol,   Al. 

(OCCURRENCE. — Aluminum  is  never  found  in  the 
metallic  state.  Of  all  the  metals  the  sources  of 
alumina  are  the  most  numerous  and  abundant. 
Its  chief  combinations  are  with  silicon  and  other  bases. 
These  substances  undergoing  atmospheric  changes 
form  clays  and  soils,  which  under  the  influence  of  heat 
and  moisture  become  fruitful.  It  would  seem  that  its 
presence  is  not  necessary  to  the  maintenance  of  animal 
or  vegetable  life,  since  no  traces  of  it  have  been  found 
in  either.  Some  of  the  compounds  of  aluminum  are 
quite  unattractive,  but  there  are  a  number  possessing 
great  hardness  and  extraordinary  beauty.  The  follow- 
ing with  their  formulae  are  a  few  examples  of  the  latter  : 

Ruby- A1203. 

Sapphire A1203. 

Garnet  .        .       (Ca  Mg  Fe  Mn)  3Al2Si3012. 

Cyanite Al2Si05. 

As  early  as  1760  Guytori  de  Morveau  called  the  sub- 
stance obtained  by  calcining  alum  alumina.  Lavoisier, 
sixteen  years  later,  suggested  the  existence  of  metal- 
lic bases  of  the  earths  and  alkalies,  and  alumina  was 
thought  to  be  an  oxid  of  a  metal  which  was  called 
aluminium  ;  and  thus  it  was  named  long  before  it  was 
isolated. 

*  Corundum,  the    ruby,  and    the    sapphire    have  the  same  chemical 
formula,  A1203. 

244 


ALUMINUM.  245 

In  1807  Sir  Humphry  Davy  tried  to  decompose 
alumina  by  means  of  an  electric  current,  and  again  to 
reduce  the  metal  by  vapor  of  potassium,  in  both  of 
which  experiments  he  failed. 

In  1827  Wohler  isolated  the  metal  by  decomposing 
aluminium  chlorid  by  potassium.  The  metal  first 
isolated  by  Wohler  was  a  gray  powder,  taking  under 
the  burnisher  the  appearance  of  a  highly  polished 
metal.  Later,  in  1845,  Wohler  obtained  the  metal  in 
small  malleable  globules  by  making  a  vapor  of  alum- 
inum pass  over  potassium  placed  in  platinum  vessels, 
and  from  these  specimens  he  was  able  to  determine  the 
properties  of  the  metal  with  some  degree  of  accuracy. 

The  credit  of  the  reduction  of  aluminum  in  a  state 
of  purity  and  the  determination  of  its  true  properties 
belongs  to  H.  St.  Clair  Deville,  who  in  1854  brought 
it  from  the  rank  of  a  mere  laboratory  curiosity  to  that 
of  the  useful  metals. 

In  1854  the  Emperor  Napoleon  III,  in  the  hope 
that  aluminum  might  be  used  in  the  construction  of 
armor  and  helmets  for  the  French  cuirassiers,  author- 
ized experiments  on  a  large  scale  to  be  carried  on  at 
his  own  expense.  In  1855  the  Emperor  put  the  neces- 
sary funds  at  the  disposal  of  Deville,  whose  experi- 
ments were  continued  for  four  months,  the  result 
being  that  in  August  of  the  same  year  aluminum  was 
placed  on  thejmarket  in  Paris  at  300  francs  a  kilo. 

The  first  article  known  to  have  been  made  of  alum- 
inum was  a  baby-rattle  for  the  infant  Prince  Imperial, 
for  which  purpose  it  was  well  fitted  on  account  of  its 
sonorousness  ;  but  application  of  the  metal  to  the 
manufacture  of  cuirasses  and  helmets  was  decided  to 
be  impracticable,  and  the  idea  was  abandoned. 


246  DENTAL   METALLURGY. 

Reduction  of  Aluminum. — A  mixture  of  the  double 
chlorid  of  aluminum  and  sodium,  or  the  double  fluorid 
of  aluminum  and  sodium  (cryolite),  is  heated  to  red- 
ness with  the  metal  sodium,  when  energetic  chemical 
action  takes  place,  during  which  chlorid  of  sodium 
is  formed  and  the  metal  aluminum  separated. 

Aluminum  may  be  separated  by  electrolysis.  The 
electric  current  from  a  ten- cell  battery,  provided  with 
carbon  poles,  is  passed  through  the  fused  salt.  The 
metal  appears  at  the  negative  pole  in  large  globules. 

In  1882  the  cost  of  aluminum  was  materially  les- 
sened by  inventions  of  Webster,  of  Birmingham, 
England.  This  inventor's  method  of  producing  the 
metal  consisted  in  reducing  sodium  compounds  in  cast- 
iron  pots  from  a  fused  bath  of  caustic  soda.  By  this 
means  the  yield  of  sodium  is  much  greater  than  by  the 
method  of  Deville,  while  the  temperature  required  in 
the  operation  is  considerably  less. 

In  1859  Mr.  Chas.  M.  Hall,  of  Ohio,  obtained 
letters-patent  for  an  electrolytic  method  which  is 
superior  to  any  that  preceded  it.  The  principal  fea- 
ture of  this  process  is  the  electric  decomposition  of 
alumina  suspended  or  dissolved  in  a  fused  bath  of  the 
salts  of  aluminum,  the  current  reducing  the  alumina 
without  affecting  its  solvent.  Mr.  Hall  has  succeeded 
in  producing  the  metal  aluminum  as  an  article  of  com- 
merce at  $2.00  per  pound. 

Aluminum  is  nearly  the  color  of  new  zinc.  It  is 
very  malleable  and  ductile,  and  admits  of  rolling  into 
thin  sheets,  or  it  may  be  drawn  into  fine  wire.  It  is 
highly  sonorous,  and  has  the  power  of  conducting 
heat  and  electricity  in  about  the  same  degree  as  silver. 
It  is  onlv  two  and  a  half  times  heavier  than  water  (four 


ALUMINUM.  247 

times  lighter  than  silver).     Its  specific  gravity  is  2.56, 
and  it  melts  at  a  red  heat. 

Aluminum  does  not  oxidize  in  air,  and  is  not 
attacked  by  sulfur  compounds.  It  is  not  attacked  by 
strong  nitric  acid,  and  is  insoluble  in  dilute  sulfuric 
acid,  but  it  may  be  readily  dissolved  in  either  dilute  or 
strong  hydrochloric  acid,  which  is  its  true  solvent. 
The  metal  is  easily  dissolved  in  solutions  of  caustic 
potash  or  soda. 

As  the  result  of  the  invention  of  the  electrical  fur- 
nace of  the  Messrs.  Cowles,  of  Cleveland,  Ohio, 
aluminum  bronze  is  made  directly  from  corundum 
(A1203).  Twenty-five  pounds  of  the  crushed  ore  is 
mixed  with  about  fifty  pounds  of  copper  and  twelve 
pounds  of  a  mixture  of  charcoal  and  electric- light 
carbon,  and  placed  in  a  rectangular  box  of  fire-brick, 
lined  with  limed  charcoal  to  prevent  loss  of  heat  by 
radiation  and  to  protect  the  fire-brick  from  disintegra- 
tion. The  charge  is  surrounded  on  all  sides  by  a  layer 
of  charcoal  to  prevent  the  alloy  from  being  contami- 
nated with  calcium  from  reduction  of  the  lime  present. 
The  cast-iron  slab  forming  the  cover  of  the  furnace  is 
then  securely  luted  on,  and  the  current  from  a  power- 
ful dynamo-electric  machine  is  passed  into  the  furnace 
by  means  of  two  large  electric-light  carbons  which 
pass  through  the  ends  of  the  furnace  and  into  its  con- 
tents. It  requires  about  five  hours  of  exposure  to  the 
intense  heat  afforded  by  the  electric  current  to  reduce 
the  aluminum  from  its  ore.  When  the  furnace  has 
cooled  sufficiently  the  product  of  the  reduction  will 
be  found  to  consist  of  about  fifty  pounds  of  a  copper 
alloy  containing  from  15  to  35  per  cent,  of  aluminum, 
which   may  be   brought  to  the    usual    10   per   cent. 


248  DENTAL     METALLURGY. 

standard  of  aluminum  bronze  by  remelting  it  with 
the  proper  proportion  of  copper. 

The  reaction  which  takes  place  in  the  process,  which 
is  aided  by  the  intense  heat  of  the  electric  current,  is 
probably  as  follows  :  The  carbon  unites  with  the  oxy- 
gen from  the  corundum,  forming  carbon  monoxid  ;  a 
small  percentage  of  the  aluminum  remains  free,  mixed 
in  small  particles  with  the  charcoal,  while  the  greater 
portion  unites  with  the  copper  to  form  the  alloy. 
Nearly  all  the  oxid  is  reduced  and  the  charcoal  is 
changed  to  graphite.  Some  of  the  aluminum  unites 
with  carbon  to  form  the  carbid  of  aluminum.  The 
fusing-point  of  10  per  cent,  aluminum  bronze  is  some- 
what below  that  of  pure  gold. 

Aluminum  may  be  melted  in  an  ordinary  clay  cruci- 
ble. No  flux  need  be  used.  Borax  is  not  only  useless 
but  is  actually  hurtful,  as  aluminum  readily  attacks  the 
glasses.  Biederman*  recommends  dipping  the  scraps 
which  are  to  be  melted  together  in  benzine  before  put- 
ting in  the  crucible.  Should  any  of  them  be  contam- 
inated with  solder  it  may  be  removed  by  immersing 
in  nitric  acid,  which  does  not  act  upon  the  aluminum. 

Annealing. — Aluminum  may  be  softened  by  heat- 
ing to  redness  and  chilling  suddenly  by  dropping  into 
water.  Richards  recommends  rubbing  the  piece  to 
be  annealed  with  tallow  and  then  heating  until  the  fat 
is  carbonized,  when  at  the  moment  the  last  trace  of 
black  disappears  from  the  metal  it  may  be  dropped 
into  water. 

Alloys. — Aluminum  forms  alloys  with  nearly  all  the 
metals.     That  with  copperf  is  the  most  important, 

*"  Aluminum:  Its  Properties,  Metallurgy,  and  Alloys."     Richards. 
f  Aluminum  bronze— alloy  of  copper  with  five  per  cent,  of  aluminum. 


ALUMINUM.  249 

and  presents  a  closer  resemblance  to  gold  than,  per- 
haps, any  other  alloy.  It  is  used  for  articles  of 
jewelry,  for  mountings  of  astronomical  instruments, 
and  for  making  balance-beams. 

German  dentists  are  now  using  aluminum  bronze  as 
a  base  for  artificial  dentures.  Professor  Sauer,  in  a 
paper  on  the  application  of  this  alloy  to  dental  pur- 
poses, says,  "That  in  the  proportion  of  Cu.  900  to 
Al.  100  it  oxidizes  but  superficially  in  the  mouth,  and 
is  as  strong  and  resistant  to  attrition  as  18- carat  gold  ; 
it  may  be  swaged  as  easily  as  20- carat  gold,  but  it 
must  be  annealed  frequently,  and  it  is  necessary  to 
carry  the  heating  almost  to  whiteness,  for  if  the  bronze 
be  merely  heated  until  it  assumes  a  dark-red  color  it 
remains  as  hard  as  before."  He  also  gives  the  point 
of  fusion  of  the  alloy  as  above  that  of  18-carat  gold, 
so  that  14-  or  18-carat  gold  solder  alloyed  with  copper 
may  be  used  upon  it  without  difficulty.  Although 
the  alloy  is  highly  recommended  by  many  German 
dentists,  the  author  does  not  hesitate  to  express  the 
opinion  that  it  will  not  find  favor  in  this  country. 

The  following  solders  are  well  adapted  to  aluminum 
bronze  : 

I.  Hard  Solder  for  10  per  cent.  Aluminum  Bronze. 

Gold 88.88  per  cent. 

Silver 4.68 

Copper 6.44        " 


100.00 


II.  Medium  Hard  Solder  for  10  per  cent.  Aluminum  Bronze. 

Gold 5440  per  cent. 

Silver 27.00        " 

Copper 18.00        " 


100.00 
17 


250  DENTAL     METALLURGY 


III.  Soft  Solder  for  Aluminum  Bronze. 

Copper  70  per  cent. ")  _ 

Tin  «  J  Brass       .     14  30  per  cent. 

Gold  .  14.30  " 
Silver  .  57.10  " 
Copper  .     14 .30 


100.00 


Aluminum  with  tin  and  zinc  forms  a  brittle  alloy, 
and  with  silver  it  yields  a  hard,  though  workable  com- 
pound. Aluminum  amalgamates  with  mercury  by  the 
assistance  of  heat,  and  at  the  boiling-point  of  mercury 
the  solution  is  very  rapid. 

Aluminum  may  be  made  to  unite  with  mercury  by 
the  intervention  of  a  solution  of  caustic  potash  or 
soda.  If  the  surface  of  the  metal  be  well  cleaned,  or 
moistened  with  the  alkaline  solution,  it  is  immediately 
melted  by  the  mercury,  but  the  affinity  of  the  alumi- 
num for  oxygen  is  greatly  increased  by  the  state  of 
fine  division,  so  that  the  amalgam  when  exposed  to 
the  air  soon  becomes  covered  with  a  white  excrescence, 
which  Watts  found  to  be  pure  alumina. 

Aluminum  is  employed  in  the  manufacture  of  very 
small  weights,  such  as  the  milligram  of  the  metric 
system, — a  use  to  which,  in  consequence  of  its  ex- 
ceedingly low  specific  gravity,  it  is  particularly  well 
adapted. 

Its  lightness,  strength,  and  resistance  to  oxygen  and 
the  sulfur  compounds,  are  properties  which  would 
seem  to  point  to  this  metal  as  a  suitable  substance  as 
a  base  for  artificial  teeth.  The  readiness,  however, 
with  which  it  is  attacked  by  alkaline  solutions  renders 
it  unfit  for  use  in  the  construction  of  a  permanent 
artificial  denture. 


ALUMINUM.  25I 

Aluminum,  notwithstanding  its  extreme  lightness, 
may  be  cast  with  great  exactness.  The  late  Dr. 
J.  B.  Bean,  who  patented  a  process  for  casting  alum- 
inum, succeeded  in  producing  castings  of  exquisite 
fineness.  Indeed,  it  may  be  stated  that  Bean  suc- 
ceeded in  overcoming  all  the  physical  difficulties  en- 
countered in  the  effort  to  render  aluminum  available 
in  prosthetic  dentistry,  but  its  susceptibility  to  the 
action  of  alkaline  solutions  finally  compelled  him  to 
abandon  it. 

Dr.  C.  C.  Carroll,  of  Meadville,  Pa.,  has  devised  a 
means  of  casting  aluminum,  which  while  much  sim- 
pler than  the  method  of  Dr.  Bean,  affords  results 
equally  good.  The  metal  is  melted  in  a  plumbago 
crucible  having  the  form  of  a  thick- walled  cylinder 
closed  at  one  end  which  serves  as  a  bottom.  A  channel 
is  formed  within  the  wall  of  the  crucible,  one  orifice 
of  which  terminates  within  at  the  side  close  to  the  bot- 
tom. Starting  from  the  orifice,  the  channel  rises  in 
the  crucible  wall  near  the  top,  making  a  sharp  return 
upon  itself,  and  descends  in  a  parallel  course  after  the 
manner  of  a  syphon,  and  makes  its  exit  at  the  base 
and  near  the  side  of  the  crucible.  Here  it  terminates 
in  an  iron  nipple  that  fits  into  a  corresponding  socket 
in  the  gate-way  of  the  molding-flask.  A  cylindrical 
plug  of  soapstone  which  fits  the  open  mouth  of  the 
crucible  is  provided  with  a  central  tube  of  brass,  to 
the  free  end  of  which  is  connected  by  a  short  length 
of  rubber  tubing  a  large  rubber  bulb.  When  the 
metal  has  been  brought  to  a  state  of  fusion  the  crucible 
is  connected  by  means  of  the  iron  nipple  at  its  base 
with  the  gate-way  of  the  flask,  which  has  been  pre- 
viously heated  to  redness,  and  the  soapstone  plug  is 


252  DENTAL    METALLURGY. 

inserted  in  the  mouth  of  the  crucible.  Compression 
of  the  air  at  this  point  by  means  of  the  rubber  bulb 
forces  the  fluid  metal  out  of  the  crucible  through  the 
syphon-like  channel  into  the- mold,  filling  the  most 
minute  lines  and  affording  an  exceedingly  fine  casting. 
Carroll  makes  the  somewhat  extraordinary  statement 
that  he  has  found  a  means  of  controlling  the  contrac- 
tion of  the  metal,  together  with  its  tendency  to  disin- 
tegrate from  exposure  to  the  fluids*  of  the  mouth,  by 
the  admixture  of  other  metals. 

Richards,  in  his  valuable  work  on  aluminum,  states 
that  to  overcome  the  difficulties  of  contraction  and 
corrosion  by  the  fluids  of  the  mouth  Dr.  Carroll  adds 
"a  little  copper,  which,  he  says,  decreases  the  con- 
traction, while  the  addition  of  some  platinum  and 
gold  renders  it  unalterable  in  the  mouth." 

Aluminum  may  be  cast  upon  plain  teeth  with  com- 
parative safety,  provided  the  metal  is  prevented  from 
overlapping  the  necks  of  the  teeth.  But  when  gum 
teeth  are  employed,  either  single  or  in  sections,  their 
fracture  is  almost  certain  to  follow  the  contraction 
incident  to  the  cooling  of  the  metal.  Specimens  of 
Dr.  Carroll's  work  have  fully  proved  this,  and  it  was 
the  one  difficulty  which  finally  defeated  Dr.  Bean's 
efforts  by  compelling  him  to  cast  his  plate  separate 
from  the  teeth.  For,  if  it  had  been  practicable  to 
cast  the  metal  directly  upon  block  teeth  without  dan- 
ger of  fracture,  the  denture  would  have  lasted  for  at 
least  six  or  eight  years  ;  but  the  necessity  of  attaching 
the  teeth  to  the  plate  by  another  metal  so  hastened 
disintegration  that  a  few  months  only  were  necessary 
to  render  the  piece  useless. 

*  Demonstration  before  dental  class,  University  of  Pennsylvania. 


ALUMINUM.  253 

There  are  two  methods  which  have  been  employed 
in  the  construction  of  artificial  dentures  of  this  metal. 
The  one  most  frequently  resorted  to  consists  in  merely 
swaging  a  plate  in  the  ordinary  way.  A  number  of 
countersunk  holes  are  then  made  along  the  part  cov- 
ering the  top  of  the  alveolar  ridge,  as  a  means  of 
fastening  the  teeth,  which  are  attached  with  rubber  or 
celluloid.  Sets  of  teeth  made  in  this  way  have  been 
known  to  do  good  service  for  eight  or  nine  years, 
but  they  showed  unmistakable  evidence  of  the  action 
of  the  oral  fluids.  In  the  second  method  the  plate 
is  cast,  but  disintegration  in  this  case  progresses  with 
much  greater  rapidity.  As  the  plate  is  cast  separate 
from  the  teeth,  and  the  latter  are  afterward  attached  by 
means  of  tin  or  an  alloy  of  tin  and  aluminum,  it  is 
probable  that  the  galvanic  action  incident  to  the  pres- 
ence of  the  two  metals  greatly  hastens  solution  of  the 
plate. 

For  some  time  the  difficulty  of  soldering  aluminum 
prevented  the  metal  from  being  applied  to  useful  pur- 
poses. The  solder  recommended  for  general  use  in 
the  manufacture  of  articles  of  ornamentation  is  com- 
posed of  copper,  four  parts  ;  aluminum,  six  parts  ; 
zinc,  ninety  parts.  The  use  of  this  requires  some 
skill  and  experience.  At  the  moment  of  fusion  small 
aluminum  tools  are  used,  the  friction  of  which  is 
necessary  to  induce  adhesion.  Borax  cannot  be  em- 
ployed as  a  flux,  as  it  is  liable  to  attack  the  metal  and 
prevent  union. 

Another  method  of  uniting  two  pieces  of  aluminum 
with  ordinary  solder  in  conjunction  with  silver  chlorid 
as  a  flux  has  recently  been  recommended  by  F.  J.  Page 
and  H.  A.  Anderson,  of  YVaterbury,  Conn.    The  finely 


254  DENTAL     METALLURGY. 

powdered  fused  silver  chlorid  is  spread  along  the  lines 
of  junction,  and  the  solder  is  melted  with  a  blow-pipe 
or  other  device.  The  union  thus  obtained  is  said  to  be 
perfectly  strong-  and  reliable.* 

The  following  alloys  are  also  used  as  solders  in 
unalloyed  aluminum  articles  of  jewelry  : 

I.  II.  III.  IV. 

Zinc  .    80  85  88  92 

Aluminum      20  15  12  8 

In  soldering  with  these  alloys  a  mixture  is  used  as 
a  flux  consisting  of  three  parts  copaiba  balsam,  one 
part  Venetian  turpentine,  and  a  few  drops  of  lemon- 
juice.     The  soldering-iron  is  dipped  into  the  mixture. 

Mr.  Wm.  Frishmuth,  of  Philadelphia,  recommends 
the  following  solders  for  aluminum,  with  vaselin  as 
the  flux  : 

Soft  Solder. 

Pure  Block  Tin  .        .        .     from  99  to  90  parts. 
Bismuth      .        .        .        .        "       1  "  10    " 

Hard  Solder. 
Pure  Block  Tin  .         .         .     from  98  to  90  parts. 
Bismuth      .        .        .        .        "      1  "    5     " 
Aluminum  .         .         .         .        "      1  "    5     " 

Schlosserf  recommends  two  solders  containing 
aluminum  as  especially  suitable  for  dental  laboratory 
use  : 

Platinum-Aluminum  Solder. 

Gold 30  parts. 

Platinum 1     " 

Silver 20     " 

Aluminum 100     " 

*  Chemical  News,  iv,  81.  t  Richards. 


ALUMINUM.  255 

Gold-Aluminum  Solder. 

Gold .50  parts. 

Silver 10      " 

Copper 10      " 

Aluminum 20      " 

O.  M.  Thowless  has  patented  the  following  solder 
for  aluminum,  and  method  of  applying  it  : 

Tin 55  parts. 

Zinc 23     " 

Silver 5     " 

Aluminum 2     " 

The  silver  and  aluminum  are  first  melted  together, 
the  tin  and  zinc  are  then  added  in  the  order  named. 
The  surfaces  to  be  soldered  are  immersed  in  dilute 
caustic  alkali  or  a  cyanid  solution,  and  then  washed" 
and  dried.  They  are  next  heated  over  a  spirit-lamp, 
coated  with  the  solder,  and  clamped  together  ;  small 
pieces  of  the  solder  being  placed  at  the  points  of  union, 
the  whole  is  then  heated  to  the  melting-point.  No 
flux  is  used. 

The  only  oxid  of  this  metal  is  alumina  (A1203).  It 
is  prepared  by  mixing  a  solution  of  alum  with  excess 
of  ammonia.  The  resulting  precipitate  (aluminum 
hydrate)  is  of  a  bulky,  gelatinous  character,  and  re- 
quires to  be  calcined  at  a  high  temperature  ;  after 
which  it  may  be  described  as  a  perfectly  white  powder, 
soluble  in  caustic  potassa  or  soda,  and  not  readily 
acted  upon  by  acids.  Corundum  and  emery  are 
nearly  pure  alumina.  The  ruby  and  sapphire  are 
also  transparent  varieties  of  alumina  in  a  crystalline 
state,  their  brilliant  colors  being  due  to  oxid  of 
chromium. 

The  only  known  sources  of  corundum   until  1869 
were  a  few  rivers  in  India,  where  it  occurred  in  crys- 


256  DENTAL   METALLURGY. 

tals  having  the  form  of  the  double  six-sided  cone. 
Its  cost  at  that  time  was  from  twelve  to  twenty-five 
cents  a  pound.  Since  that  date,  however,  it  has 
been  discovered  in  inexhaustible  quantities  in  Georgia, 
North  Carolina,  and  Pennsylvania.  At  present  it  can 
be  bought  at  the  mines  at  $10  per  ton. 

For  the  discrimination  of  the  salts  of  aluminum, 
see  any  of  the  recent  works  on  chemistry. 


CHAPTER   XIX. 

LEAD. 
Atomic  Weight,  207.     Symbol,  Pb  (Plumbum  . 

THE  reduction  of  lead  is  effected  in  a  reverberatory 
furnace,  in  which  the  broken  lead  ore  (galena) 
is  roasted  at  a  dull-red  heat,  by  which  means 
the  sulfid  becomes  oxidized  and  converted  into  sul- 
fate. At  this  stage  of  the  operation  the  contents  of 
the  furnace  are  thoroughly  mixed  and  the  tempera- 
ture raised,  which  causes  the  sulfid  and  sulfate  to 
react  upon  each  other,  producing  sulfurous  oxid  and 
metallic  lead. 

Lead  is  the  softest  metal  in  common  use,  and  may 
be  said  to  be  the  least  tenacious.  In  fusibility  it  also 
surpasses  all  other  metals  commonly  employed  in  the 
metallic  state  except  tin,  the  fusing-point  of  lead  being 
6170  F.  =  325°  C.  It  is  quite  malleable  and  ductile, 
and  will  admit  of  being  rolled  into  thin  sheets  or  foil, 
in  which  form  it  was  at  one  time  much  used  in  filling 
teeth.  Its  chief  use  in  the  dental  laboratory  consists 
in  the  formation  of  counter-dies. 

Alloys. — Lead  unites  with  tin   in  all   proportions, 
the  resulting  alloys  being  more  tenacious  and  fusible 
than  either  constituent.     By  the  addition  of  bismuth 
the  fusing-point  is  reduced  below  the  boiling-point  of 
water. 

Lead  amalgamates  readily  with  mercury,  conden- 
sation accompanying  the  union.  The  noble  metals 
are  all  rendered  brittle  and  unworkable  by  the  pres- 

257 


258  DENTAL     METALLURGY. 

ence  of  lead.  There  are  some  properties  peculiar  to 
alloys  of  lead  and  silver  which  are  turned  to  advan- 
tage in  the  separation  of  silver  from  lead  when  it 
occurs  as  a  native  alloy.  Lead  combined  with  a  con- 
siderable quantity  of  silver  will  remain  fluid  at  a 
lower  temperature  than  other  specimens  containing  a 
smaller  percentage,  thus  affording  an  opportunity  for 
the  poorer  lead  to  crystallize,  when  it  is  ladled  out.* 

The  smallest  proportion  of  lead  in  gold  will  greatly 
impair  the  ductility  of  the  latter.  Makins  states  that 
"  Hatchett  found  that  y-gVo"  of  lead  destroyed  the 
coining  qualities  of  gold."  Gold  reduced  to  standard 
fineness  by  lead  is  light-yellow  in  color,  and  quite 
brittle.  The  contents  of  the  dentist's  gold-drawer 
are  always  liable  to  contamination  by  small  pieces  of 
lead,  the  latter  being  much  used  in  the  form  of  thin 
sheets  in  the  making  of  patterns  by  which  the  gold  or 
silver  plate  is  cut.  As  the  working  qualities  of  the 
precious  metals  are  seriously  impaired  by  its  presence, 
means  should  be  instituted  to  insure  its  complete  re- 
moval. This  may  be  accomplished  by  cupellation,  or 
by  melting  the  gold  or  silver  in  a  crucible,  and  adding 
nitrate  of  potassium  when  the  point  of  complete 
fusion  has  been  reached. 

Lead  and  platinum,  like  tin  and  platinum,  appear 
to  possess  considerable  affinity  for  each  other,  and  an 
alloy  of  the  two  can  be  formed  at  a  comparatively 
low  temperature. 

An  alloy  of  lead  and  platinum  is  very  hard  and 
brittle.  With  palladium  also  lead  forms  a  very  hard 
and  brittle  alloy. 

*  See  chapter  on  "  Silver." 


LEAD.  259 

The  most  valuable  alloys  of  lead  are  those  which 
it  forms  with  tin,  antimony,  and  bismuth,  constituting 
solders,  pewter,  type-metal,  etc. 

For  the  discrimination  of  lead,  the  student  is  re- 
ferred to  Fownes's  or  other  standard  works  on 
chemistry. 


CHAPTER   XX. 

TIN. 
Atomic  Weight,  118.    Symbol,  Sn  (Stannum). 

THE  metal  tin  has  been  known  for  probably  three 
thousand  years.  It  is  found  in  all  parts  of  the 
world,  chiefly  as  oxid.  In  reducing  the  ore  it  is 
first  powdered  and  roasted  to  free  it  of  sulfur  and 
arsenic.  It  is  then  exposed  to  a  high  temperature  with 
charcoal,  and  the  metal  is  thus  liberated. 

Pure  tin  is  white  in  color,  and  is  perfectly  soft  and 
malleable.  It  has  a. density  of  7.3,  and  its  fusing- 
point  is  458. 6°  F.  (2370  C).  It  is  but  slightly  acted 
upon  by  air,  but  when  heated  much  above  its  melting- 
point  it  oxidizes  freely,  and  is  converted  into  a  yel- 
lowish-white powder, — the  well-known  polishing  - 
putty.  The  action  of  nitric  acid  upon  tin  is  to  con- 
vert it  into  a  white  hydrated  dioxid.  It  is  dissolved 
by  hydrochloric  acid,  assisted  by  heat,  and  forms 
stannous  chlorid.  Nitro-hydrochloric  acid  acts  upon 
tin  with  much  energy,  converting  it  into  stannic 
chlorid. 

Alloys. — Tin  is  readily  dissolved  in  mercury  (see 
page  50).  With  silver  it  forms  a  malleable  alloy, 
which  is  considerably  harder  than  tin.  The  late  Dr. 
Bean  used  tin  alloyed  with  a  small  percentage  of  silver 
for  lower  sets,  which  he  cast  directly  upon  the  teeth 
after  the  ordinary  cheoplastic  method. 

Alloys  of  tin  and  silver,  in  which  the  former  is 
slightly  in  excess,  are  much  used  as  amalgam  alloys. 
260 


TIN.  26l 

Tin  10,  silver  8,  gold  1,  is  also  frequently  employed 
in  filling  teeth  ;  and  tin  10,  silver  8,  gold  i,  copper  1, 
has,  according  to  Fletcher,  been  largely  used  as  ' '  gold 
and  platinum"  amalgam.  It  is  stated  that  from  5  to 
7  per  cent,  of  copper  has  the  property  of  replacing 
platinum  in  amalgams,  conferring  the  quick-setting 
quality  claimed  for  platinum.'^ 

Dr.  G.  F.  Reese  has  formed  an  alloy  for  a  base  for 
artificial  dentures,  composed  of  20  parts  of  tin,  1  of 
gold,  and  2  of  silver. f  This  is  cast  directly  upon  the 
teeth,  the  process  being  similar  to  the  cheoplastic 
method. 

The  alloys  which  have  been  used  in  the  cheoplastic 
process  are  chiefly  composed  of  tin,  silver,  bismuth, 
and,  in  some  instances,  cadmium  and  antimony. 

According  to  Makins,  gold  and  tin  form  a  malleable 
alloy,  X  and  gold  reduced  to  standard  by  pure  tin  re- 
tains its  malleability. 

Tin  and  platinum  in  equal  proportions  afford  a  hard 
and  quite  brittle  alloy,  fusible  at  a  comparatively  low 
temperature.  When  it  is  remembered  that  the  fusing- 
points  of  these  metals  almost  represent  extremes  of 
temperature,  it  would  seem  that  their  union  must  be 
attended  with  difficulty,  but,  as  has  already  been 
stated,  1 1  it  is  probable  that  some  affinity  exists  between 
the  two,  as  platinum  is  readily  dissolved  by  and  alloys 
with  the  fused  tin. 


*  T.  Fletcher. 

t  Alloys  and    Amalgams  Chemically  Considered,"  J.   Morgan   Howe, 
M.D. 

%  A  precipitated  alloy  of  gold  and  tin,  having  the  form   of  a  black  pow- 
der, may  be  formed   by  acting  upon  a  concentrated  solution  of  tnchlorid 
of  gold  with  stannous  chlorid. 
See  chapter  on  "  Alloys." 


262  DENTAL   METALLURGY. 

With  palladium  tin  is  said  to  form  a  brittle  alloy. 

With  lead  tin  forms  the  chief  part  in  the  alloys 
used  for  soft-soldering,  and  in  the  compounds  known 
as  pewter  and  Britannia-metal.  Tin  solders  are  com- 
posed of  two  parts  of  tin  to  one  of  lead.  Pewter 
consists  of  four  parts  of  tin  to  one  of  lead,  while 
Britannia-metal  is  formed  by  the  addition  of  small 
quantities  of  antimony  and  copper. 

Alloys  of  tin  and  lead  are  harder  and  tougher  than 
either  metal  singly,  and  they  are  more  fusible  than 
the  mean  of  their  constituents.  The  addition  of  bis- 
muth to  such  an  alloy  lowers  the  melting-point  to  a 
remarkable  degree,  and  the  fusing-point  is  still  further 
reduced  by  the  addition  of  cadmium.  Thus,  an  alloy 
composed  of  15  parts  of  bismuth,  8  of  lead,  4  of  tin, 
and  3  of  cadmium,  fuses  at  1450  F.  =  630  C. 

Dr.  C.  M.  Richmond  used  a  fusible  alloy  in  crown- 
and  bridge-work  which  he  states  is  as  hard  as  zinc, 
andean  be  melted  at  1500  F. ,  and  poured  into  a 
plaster  impression  without  generating  steam.  The 
formula  of  this  alloy  is  as  follows  : 

Tin 20  parts  by  weight. 

Lead 19     "       " 

Cadmium  .         .         .        .  13     "       " 

Bismuth     .        .        .        .  48     "      " 

The  following  fusible-metal  alloys  are  also  suitable 
for  the  purpose  : 


in. 

Lead. 

Bismuth. 

Melting-point  of  Alloy 
Fahr. 

I 

2 

2 

236° 

5 

3 

3 

202° 

3  5  8  1970 

Dr.     George   W.    Melotte,    of  Ithaca,    N.    Y.,    to 


TIN.  p  263 

whom  is  due  the  credit  of  having  introduced  the  use 
of  fusible  metal  and  the  compound  called  "  moldine" 
into  bridge-work,  uses  an  alloy  of — 

Tin,  5, 

Lead,        3, 
Bismuth,  S. 

Moldine,  of  which  Melotte  forms  his  matrix  for 
casting,  is  a  compound  of  potter's  clay  and  glycerin. 
The  alloy  known  as  "Wood's  metal,"  occasionally 
employed  by  dentists  in  replacing  teeth  on  vulcanite 
plates,  is  composed  of  7  parts  of  bismuth,  6  of  lead, 
and  1  of  cadmium,  and  fuses  at  1800  F.  =  820  C,  a 
point  much  below  the  boiling-point  of  water.  In  re- 
placing a  broken  tooth  by  means  of  Wood's  metal 
the  usual  dovetail  is  cut  in  the  rubber  plate  with  a  fine 
saw,  the  tooth  is  fitted  to  its  place,  and  the  fusible 
alloy  is  packed  in  with  a  spatula  heated  in  a  spirit- 
lamp. 

Lead  75,  tin  5,  and  antimony  20  parts,  is  the  com- 
position of  the  best  form  of  type-metal. 

With  copper  tin  affords  a  number  of  very  useful 
alloys.  Bell-metal  is  formed  of  78  parts  of  copper  to 
2  of  tin.  Gun-metal  is  formed  of  90  per  cent,  of 
copper  to  10  per  cent,  of  tin.  Speculum-metal  is 
formed  of  6  parts  of  copper,  3  of  tin,  and  1  of  arsenic. 

Babbitt-metal,  named  after  Isaac  Babbitt,  of  Bos- 
ton, Mass.,  is  an  alloy  consisting  of  9  parts  tin,  10  parts 
copper,  used  for  journal  boxes  {vide  patent  1839). 
Many  modifications  have  since  been  made  in  this  alloy, 
but  the  term  is  still  applied  to  any  white  alloy  em- 
ployed in  the  construction  of  bearings,  to  distinguish 
it  from  the  "  bronzes" and  "  brasses." 

Mr.    Fletcher   recommends  an   alloy    of  copper   4 


264  DENTAL   METALLURGY. 

pounds,  Banca  tin  96  pounds,  Regulus  antimony  8 
pounds.  This  alloy  is  said  to  be  nearly  as  hard  as 
zinc,  while  its  shrinkage  is  much  less.  These  quan- 
tities, together  with  the  low  temperature  at  which  it 
fuses,  entitle  it  to  a  place  in  the  dental  laboratory  for 
the  preparation  of  dies  and  counter-dies. 

Dr.  L.  P.  Haskell  recommends  the  formula,  tin 
72.72,  copper  9.09,  antimony  18.18. 

Bronze  is  an  alloy  of  copper  and  tin,  and  some- 
times zinc.  It  is  affected  by  changes  of  temperature 
in  a  manner  precisely  the  reverse  of  that  in  which 
steel  is  affected,  becoming  soft  and  malleable  when 
quickly  cooled,  and  hard  and  brittle  when  allowed  to 
cool  slowly.  The  art  of  making  bronze  was  practiced 
before  any  knowledge  of  the  working  of  iron  existed, 
and  it  was  used  at  a  .very  early  period  in  the  manu- 
facture of  weapons. 

Commercial  tin  is  liable  to  contain  minute  quan- 
tities of  lead,  iron,  copper,  arsenic,  antimony,  bis- 
muth, etc.  Pure  tin  may  be  precipitated  in  crystals 
by  the  feeble  galvanic  current  excited  by  immersing  a 
plate  of  tin  in  a  strong  solution  of  stannous  chlorid. 
Water  is  carefully  poured  on  so  as  not  to  disturb  the 
layer  of  tin  solution.  The  pure  metal  will  be  de- 
posited on  the  bar  of  tin  at  the  point  of  junction  of 
the  water  and  the  metallic  solution. 

Perfectly  pure  tin  may  also  be  obtained  by  dis- 
solving commercial  tin  in  hydrochloric  acid,  by  which 
it  is  converted  into  stannous  chlorid.  After  filtering, 
this  solution  is  evaporated  to  a  small  bulk,  and  treated 
with  nitric  acid,  which  instantly  converts  the  stannous 
chlorid  into  stannic  oxid.  This  is  thoroughly 
washed  and  dried,  and  exposed  to  red  heat  in  a  cruci- 


TIN.  265 

ble  with  charcoal.  A  button  of  pure  tin  will  be  found 
at  the  bottom  of  the  crucible. 

Pure  tin  in  the  form  of  foil  is  frequently  used  in  fill- 
ing teeth,  for  which  purpose  it  doubtless  ranks  next 
to  gold.  Tin  foil  is  also  employed  in  connection  with 
non-cohesive  gold  in  filling  approximal  surfaces  of 
cavities  in  bicuspids  and  molars.  Two  sheets  of  foil, 
one  of  gold  and  the  other  of  tin,  are  placed  together 
and  made  into  mats  or  cylinders.  These  are  carefully 
packed  against  the  cervical  margins  of  the  cavity. 
The  frequent  failure  of  ordinary  gold  fillings  at  this 
point  has  led  some  practitioners  to  entertain  the  theory 
that  between  the  tooth-substance  and  the  gold  there  is 
galvanic  action,  to  which  the  lime-salts  of  the  tooth 
yield,  and  that  by  the  combination  of  two  metals, 
whether  tin  and  gold  or  amalgam  and  gold,  the  gal- 
vanic action  is  confined  to  the  metals,  the  tooth-sub- 
stance being  thus  protected. 

The  appearance  of  a  filling  formed  of  tin  and  gold 
would  seem  to  confirm  this  theory,  as  it  soon  becomes 
dark  in  color,  and  presents  a  surface  resembling 
amalgam,  but  it  effectually  protects  the  margins  from 
decay.* 

Tin  having  but  slight  affinity  for  sulfur,  is  largely- 
used  in  the  formation  of  models  in  the  construction  ot 
vulcanite  dentures,  and  tin  foil  forms  the  best  coating 
for  plaster  casts  in  the  vulcanizing  process. 

The  manufacturers  of  miscellaneous  rubber  articles 
do  not  use  plaster  in  forming  the  matrix  in  which  the 
rubber  is  packed  before  vulcanizing  ;  having  long 
since  discovered  that  contact  with  plaster  lessens  the 

*  Prof.  James  Truman,  Report  of  Proceedings  of  Odontological  Society 
of  Pennsylvania,  November,  1881. 

18 


266  DENTAL   METALLURGY. 

toughness  and  elasticity  of  the  indurated  rubber, 
they  therefore  form  every  matrix  of  sheet  tin,  which 
is  placed  in  a  suitable  iron  box  and  covered  tightly 
with  dry  powdered  soapstone  (steatite). 

Dr.  J.  S.  Campbell,  who  introduced  the  vulcanizer 
known  as  the  ' '  New  Mode  Heater, ' '  described  a  means 
whereby  all  parts  of  the  matrix  contained  in  the  flask 
in  constructing  rubber  dentures  could  be  covered  by 
sheet  tin,  so  that  after  vulcanizing  and  the  removal  of 
the  sheet  tin  or  foil  the  surface  of  the  rubber  would 
be  found  to  be  smooth  and  highly  polished,  and  if 
the  ' '  waxing  up' '  had  been  carefully  done,  little  or 
no  filing  and  scraping  were  needed.  The  method 
demonstrated  by  Dr.  Campbell  possessed  precision  and 
saved  labor,  and  it  is  to  be  regretted  that  it  was  not 
generally  adopted  by  mechanical  dentists,  who  have 
not  improved  to  any  extent  upon  the  slovenly  methods 
employed  in  1858,  when  indurated  rubber  was  first 
employed  in  dental  practice. 

When  tin  foil  is  used  as  a  coating  for  plaster  casts  in 
rubber  work,  the  foil  may  be  removed  with  the  finger- 
nail if  the  surface  of  the  plaster  model  was  smooth 
and  hard,  and  this  condition  of  the  plaster  surface  can 
be  obtained  by  using  nothing  as  a  coating  for  the  plas- 
ter impression  but  sandarac  varnish,  and  carefully 
avoiding  the  use  of  oil  or  solutions  of  soapas'a  means 
of  separating  the  impression  from  the  model.  If, 
however,  in  consequence  of  the  roughened  surface  of 
the  plaster  cast  the  tin  foil  adheres  so  tenaciously  that 
it]  cannot  be  removed  except  by  means  of  a  solvent, 
hydrochloric  acid  is  the  only  one  that  will  accomplish 
that  end  without  injury  to  the  rubber.  Both  nitric 
and  nitro-hydrochloric  acids  should  be  avoided,   as 


TIN.  267 

they  attack  indurated  rubber  with  more  or  less  en- 
ergy. 

Lower  vulcanite  dentures  may  be  loaded  with  tin  to 
give  them  additional  weight,  and  by  lessening  the 
quantity  of  rubber  prevent  the  occurrence  of  porosity 
during  the  process  of  vulcanizing. 

Solve7its. — Tin  is  readily  dissolved  by  either  of  the 
three  mineral  acids.  Sulfuric  acid  converts  it  into 
stannic  sulfate.  Tin  dissolved  in  hydrochloric  acid 
forms  stannous  chlorid.  By  the  action  of  dilute  nitric 
acid  tin  is  not  dissolved,  but  is  converted  into  stannic 
oxid,  which  settles  to  the  bottom  of  the  vessel  as  a 
white  powder.  This,  when  rendered  anhydrous  by 
heating  to  redness,  affords  the  well-known  polishing- 
powder  called  "  polishing-putty." 

Chlorids. — There  are  two  chlorids  of  tin, — stannous 
chlorid  or  protochlorid  of  tin  (SnCl2),  and  stannic 
chlorid  or  bichlorid  of  tin  (SnClJ.  Stannous  chlorid 
is  prepared  by  dissolving  tin  in  hydrochloric  acid,  the 
action  being  assisted  by  gentle  heat.  Stannic  chlorid 
is  obtained  by  dissolving  tin  in  nitro- hydrochloric  acid 
(aqua  regia).  These  two  compounds  of  tin  are 
employed  in  the  preparation  of  purple  of  Cassius,  in 
which  process  stannous  chlorid  is  added  to  a  mixture 
of  stannic  chlorid  and  trichlcrid  of  gold  (see  page 
149). 

For  other  compounds  of  tin,  see  works  on  chemistry. 

Discrimination. — Tin  is  detected  before  the  blow- 
pipe by  fusing  the  compound  under  examination  on 
charcoal  with  sodium  carbonate,  when  a  bead  of  the 
metal  is  obtained.  From  a  tin  solution  caustic  potash 
and  soda  precipitate  a  white  hydrate,  soluble  in  excess. 
Ammonia  affords  a  similar  precipitate,  not  soluble  in 


268  DENTAL   METALLURGY. 

excess.     Hydrogen  sulfid  and  ammonium  sulfid  throw 
down  a  dark-brown  precipitate  of  monosulfid.     Tri- 
chlorid  of  gold  added  to  a  dilute  solution  of  stan- 
nous chlorid  causes  a  purple  precipitate  (purple  of 
Cassius). 


CHAPTER     XXI. 

ELECTRO-METALLURGY. 

THE  origin  of  electro-metallurgy  was  undoubtedly 
due  to  the  early  experiments  of  Wollaston  and 
Davy,  while  the  credit  of  its  development  belongs  to 
the  late  Professor  Daniell,  who  devised  the  particular 
form  of  battery  which  bears  his  name.  A  Daniell 
cell  consists  of  a  copper  vessel  containing  a  saturated 
solution  of  sulfate  of  copper.  In  this  is  placed  a 
porous  cylinder  containing  dilute  sulfuric  acid.  A 
rod  of  amalgamated  zinc  is  immersed  in  the  acid,  and 
on  the  two  metals  being  connected,  electrical  action  is 
immediately  set  up,  and  the  zinc,  which  forms  the 
positive  element,  is  dissolved,  with  formation  of  sul- 
fate of  zinc  ;  the  sulfate  of  copper  is  reduced,  and 
the  metallic  copper  is  deposited  upon  the  surface  of 
the  copper  vessel,  which  forms  the  negative  element 
of  the  combination.  It  was  observed  that  the  copper 
thus  deposited  took  the  exact  shape  of  the  surface  on 
which  it  was  thrown,  presenting  a  faithful  counterpart 
of  the  slightest  indentation  or  irregularity.  De  la 
Rue  called  attention  to  this  fact  in  a  paper  published 
in  1836,  but  no  practical  application  was  made  of  it 
until  1839,  when  Professor  Jacobi,  of  St.  Petersburg, 
published  his  discovery  of  a  means  of  producing 
copies  of  engraved  copper  plates  by  the  agency  of 
electricity. 

In   1840  Mr.    Murray  announced  that  an    electro- 
deposit  of  metal  could  be  formed  upon  almost  any 

269 


270  DENTAL    METALLURGY. 

material,  provided  its  surface  was  rendered  a  conductor 
of  electricity  by  a  thin  coating  of  graphite  (black  lead). 
Instead  of  copying  the  object  in  a  metallic  medium, 
it  is  only  necessary  to  take  a  cast  in  plaster  of  Paris, 
wax,  gutta-percha,  or  any  convenient  material,  and 
then  to  coat  the  surface  with  finely-powdered  graphite 
applied  with  a  camel' s-hair  pencil. 

The  Gramme  machine,  a  modification  of  the  mag- 
neto-electric apparatus,  consists  of  a  ring  of  soft  iron 
carrying  a  number  of  coils  of  insulated  copper  wire, 
caused  to  rotate  between  the  poles  of  a  fixed  horse- 
shoe magnet.  The  currents  induced  in  the  coils  are 
collected  by  two  metallic  disks,  whence  they  may  be 
drawn  off  for  use  in  electro-deposition.  The  core 
being  circular,  the  magnetization  proceeds  contin- 
uously, affording  a  uniform  current.  Both  poles  of 
the  magnet  are  used,  producing  simultaneously  two 
opposite  continuous  currents.  These  and  similar 
sources  of  electricity  enable  the  electro-metallurgist 
to  deposit  a  metal  upon  a  matrix  or  to  coat  one  metal 
with  another. 

The  art  of  electro-metallurgy  is  divided  into  two 
branches,  electrotypy  and  electro-plating.  In  the 
former  the  reduced  metal  is  separated  from  the  mold 
on  which  it  is  deposited,  forming  a  distinct  work  of 
art,  while  in  the  latter  the  deposited  metal  forms  an 
inseparable  part  of  the  plated  object.  Electrotypy  is 
employed  in  producing  copper  duplicates  of  engrav- 
ings on  wood  and  of  any  kind  of  type-matter  for 
printers'  use.  A  cast  of  the  object  is  first  taken  in 
wax  or  gutta-percha,  and  the  surface  of  this  mold  is 
brushed  over  with  black-lead,  and  it  is  then,  by  means 
of  a  wire,  suspended  in  a  bath  of  sulfate  of  copper 


ELECTRO-METALLURGY.  271 

connected  with  a  battery.  Since  the  introduction  of 
dynamic  electricity,  perfect  copies  may  be  produced 
in  an  hour  or  two,  and  the  deposited  metal  made  thick 
enough  to  stand  any  reasonable  amount  of  wear. 

Electro-plating,  by  which  silver  is  deposited  on  the 
surfaces  of  objects  of  copper,  brass,  or  German-silver, 
was  introduced  soon  after  the  discovery  of  the  art  of 
electro- metallurgy.  The  article  to  be  plated  must 
first  be  thoroughly  cleansed  by  immersing  it  in  a  hot 
solution  of  caustic  potash,  and  also  by  means  of  the 
"scratch-brush,"  and  sometimes  by  "pickling"  it  in 
a  bath  of  nitric  and  other  acids.  The  surface  is  then 
given  a  thin  film  of  mercury,  by  washing  the  article 
with  a  solution  of  mercuric  nitrate.  This  is  called 
"quicking."  The  article  is  then  rinsed  with  water 
and  transferred  to  the  silver  bath,  which  consists  of  a 
solution  of  cyanid  of  silver  in  cyanid  of  potassium, — a 
very  poisonous  compound.  Plates  of  silver  are  sus- 
pended from  a  rectangular  frame  connected  with  the 
positive  pole,  while  the  articles  to  be  plated  are  sus- 
pended by  wires  attached  to  the  negative  pole.  The 
quantity  of  silver  to  be  deposited  depends  upon  the 
requirements  of  the  case.  One  ounce  of  silver  per 
square  foot  forms  for  ordinary  purposes  a  sufficiently 
heavy  coating. 

Electro-gilding  is  effected  by  means  similar  to  those 
employed  in  electro-silvering.  The  solution  employed 
is  generally  the  double  cyanid  of  gold  and  potas- 
sium, and  it  is  used  hot,  the  temperature  ranging 
from  1300  F.  to  2120  F.,  according  to  the  ideas  of  the 
operator. 

Nickel-plating  was  first  introduced  in  1869,  by  Dr. 
Isaac  Adams,  of  Boston,  who  patented  a  process  for 


272  DENTAL   METALLURGY. 

depositing  nickel  from    solutions  of  double  salts  of 
sulfate  of  nickel  and  ammonium. 

Iron  may  also  be  deposited  from  the  double  sulfate 
of  iron  and  ammonium.  Practical  application  is  now 
made  of  this  discovery,  and  engraved  copper  plates 
are  found  to  be  much  more  durable  when  faced  with 
electro-deposited  iron.  Plates  for  printing  bank-notes 
are  sometimes  treated  in  this  way. 


INDEX. 

PAGE 

Acid,  nitric,  action  of  on  silver 177 

nitric,  in  the  quartation  process  of  refining  gold 125 

nitro-hydrochloric  (aqua  regia) 132 

sulfuric,  action  of  on  silver 170 

sulfuric,  in  the  quartation  process  of  refining  gold...  125 

Acids,  action  of  on  alloys 45 

Agents  which  may  volatilize  a  metal 33 

Alloying,  influence  of. 32,  39 

the  purposeof 35 

Alloys 34 

as  definite  compounds 36 

color  of 3S 

composition  of 41 

conductivity  of. 26 

decomposition  of. 42 

density  of. 37 

for  solders 35 

fusibility  of 40 

influence  of  constituent  metals  in 42 

liquation  of 43 

malleability,  ductility,  and  tenacity  of. 39 

Matthiesen's  definition  of. 37 

of  gold  employed  in  dentistry  as  solders 141 

of  silver  employed  in  dentistry  as  solders 185 

oxidizability  of 45 

preparation  of 44 

properties  of 35 

specific  gravity  of. 38 

study  of • 34 

table  of 41 

temper  of 44 

Von  Eckart ' s 1 84 

Alumina 244 

Aluminum 244 

273 


274  INDEX. 

PAGE 

Aluminum,  alloys  of 248 

annealing 248 

bronze 41,  249 

"      preparing,  Cowles's  method  of 247 

"      solders  for 249,  250 

casting,  Bean's  method  of 251 

casting,  Carroll's  method  of. 251 

Page's  solder 255 

reduction  of. 245 

solder  for 253,  254,  255 

solder  for  aluminum  bronze 249 

solvents  of. 247 

swaged  plates  of. 253 

Amalgams 46 

composition  of  some  of  the  well-known 71,     72 

definition  of. 46 

discoloration  of ...     48 

expansion  of. 47 

experiments  with 49 

for  dental  purposes 46 

formation  of. 46 

forming  alloys  for 50,     59 

formula  for,  Dr.  Ambler  Tees's 73 

"    Dr.  L.Jack's 61,62,63,     64 

influence  of  different  metals  in..... 50 

introducing  fillings  of. 57 

mixing,  methods  of. 56 

palladium 65,     66 

platinum 60 

qualitative  and  quantitative  examinations  of 66 

quantity  of  mercury  to  be  used  in 55 

shrinkage  of. 47 

Sullivan's 66,  229 

zinc 61 

Ammonium 19 

Argentiferous  galena 173 

Argentum  (silver) 169 

Arsenic  in  alloys 43 

Bloxam's  classification  of. 18 


INDEX.  275 

PAGE 

Arsenic,  odor  of 20 

Assay  of  amalgam  alloys 66 

Assaying  gold 157 

Atomic  weights  of  metallic  elements 14,  16 

Auric  chlorid 149 

oxid 148 

iodid 149 

silicate 153 

sulfid 149 

Aurous  chlorid 149 

Aurum  (gold) 116 

Babbitt-metal 263 

Beating  gold 165 

Bellows  for  blow-pipe 79,  80,     81 

Bessemer  steel 211 

Black-lead  crucibles 90 

Blast  furnaces 86,  87,     8S 

Blistered  steel 210 

Blow-pipes 76,  77,  78,     79 

hot-blast 79 

Knapp's S2 

oxyhydrogen 190,  191,   192 

supports  for  use  with 97 

Bone-ash  cupels 175 

Borax 96 

Brass 23S 

Britannia-metal... 262 

Brittle  gold,  treatment  of. 129 

Bromids,  metallic .• 104 

Bronze 264 

Cadmium 242 

precipitation  of 67 

Calamine 235 

Calomel 217 

Carbon,  proportion  of,  in  cast  iron  and  steel 210 

Case-hardening 216 

Cast  iron 209 


276  INDEX. 

PAGE 

Cast  steel 210 

Charcoal  as  a  reducing  agent 111 

Chlorids,  metallic 103,  in 

preparation  of 104 

reduction  of. •• in 

Cinnabar 217 

Coke  for  supports  in  soldering 99 

Color  of  metals • 19 

influence  of  alloying  on 38 

Copper 227 

alloys  of . 231 

amalgam 229 

as  a  constituent  in  amalgams. 229 

discrimination  of 233 

properties  of. 228 

solvents  of • 228,  229 

value  as  a  therapeutic  agent 231 

Copper  steel 212 

Corundum 255 

Crucibles ••  89 

Crystallization 31 

Cupellation.... 175,  176 

Cupels 175 

Cyanids,  metallic 103 

Dental  alloy 183 

Ductility  of  metals 27,     28 

influence  of  alloying  on = - 39 

Electro-deposit  of  metals 269 

Electrolysis 115 

Electro-metallurgy..... 269 

Electrotypy,  origin  of 269 

Elements,  metallic 13 

Emery 255 

Flame,  blow-pipe,  management  of 76 

Fletcher's  hot-blast  blow-pipe 78,     79 

Fluorids,  metallic 105 


INDEX.  -   277 

PAGE 
Flu* 151,      152 

Forging  platinum 193 

Fulminating  gold J54 

silver 179 

Furnace,  lime,  for  melting  platinum 192 

Cowles's  electrical - 109,  247 

Furnaces 85,  86,  87,     88 

Fusible  alloys  used  in  bridge-work 262,  263 

Fusible  metal 262,  263 

Fusing-points  of  metals 21 

Galena,  argentiferous. 173 

Gauge-plate 93 

Gold 116 

alloys  of 136 

chlorids  of. 148 

clasps  for  partial  artificial  dentures 140 

coin  136,   137 

compounds  of 148 

containing  iridium 130 

discrimina tion  of 1 54 

foil,  cohesive • 163 

foil,  non-cohesive 161 

in  combination  with  tin  in  filling  teeth 265 

Lamm's  shredded 134 

native,  formsof. 121 

oxids  of. 14S 

precipitants  for 133 

precipitated,  different  forms  of 134 

precipitation  of,  by  ferrous  sulfate 135 

precipitation  of,  by  oxalic  acid 134 

precipitation  of,  by  sulfurous  acid 135 

properties,  occurrence,  and  distribution  of 117 

pure,  preparation  of 130 

quartation  process  of  refining 124 

reducing,  to  a  higher  or  lower  carat 143 

refining 124 

swaging,  when  alloyed  with  platinum 140 

volatilizing 33,  120 


278  INDEX. 

PAGE 

Gold,  Watts's  crystal 136 

welding  properties  of 119 

Gum  frit 149 

Gun-metal 263 

Hot-blast  blow-pipes 79 

Hydrogenium 18 

Ingot-molds 92 

Iodids,  metallic 105 

Iridium 199 

Iron 207 

compounds  of 209 

fusing-point  of 208 

meteoric 207 

native - •  207 

properties  of • 207 

solvents  of 208 

Knapp's  oxyhydrogen  blow-pipe 82 

Lamps,  soldering 75 

Lead 257 

alloys  of 257 

amalgamation  of 257,  258 

desilvering  of '• 173,  258 

discrimination  of. 259 

properties  of 257 

reduction  of. 257 

Magnetic  iron 208 

Malleability . 27,  39 

influence  of  alloying  on 39 

Melotte's  fusible  metal 262 

Mercury 217 

adulterations  of 218 

chlorids 222 

compounds 222 


INDEX.  279 

PAGE 

Mercury,  discrimination  of 225 

filtration  of 220 

native 217 

occurrence  of 217 

ores,  reduction  of 217,  218 

oxids  of. 222 

properties  of 220 

pure,  to  obtain 219 

purification  of,  by  digestion 220 

purifying,  Priestley's  plan  of. 220 

quantity  of,  to  be  used  in  amalgams 55 

redistillation  of- 218 

sources  of 217 

sulfids  of 222,  223 

Metallic  bromids 104 

Metallic  chlorids 103 

cyanids 103 

fluorids 105 

iodids 105 

oxids 105 

sulfids 10S 

Metallurgy,  definition  of... 9 

Metals,  base 16 

capacity  for  heat  of 22 

color  of 19 

conduction  of  electricity  of 25 

conduction  of  heat  of 25 

effects  of  alloying  on 35 

elasticity  and  sonorousness  of 32 

expansion  of,  by  heat 23 

fusibility  of 20 

fusing-points,  table  of 21 

luster 19 

malleability,  ductility,  and  tenacity  of. 27 

modes  of  melting 74 

noble 16 

odor  and  taste  of 20 

properties  of 17 

tableof 14 


280  INDEX. 

PAGE 

Metals,  volatility  of. 32,  33 

volatility  of,  agents  which  induce 33 

Moldine 263 


Nickel-plating 27 


Oxids,  metallic 105 

reduction  of 113 

Palladium 202 

alloys  of. 203 

amalgams 65,  66,  204 

cost  of -. 205 

discrimination  of 206 

influence  of,  in  amalgams  65,  204,  205 

properties  of 203 

sources  of- •  ••• 202 

Pewter 259 

Phosphor-bronze 43,  232 

Phosphor-iridium 200 

Platinum 187 

alloys  of 194 

amalgamation  of.... 194,  195 

associate  metals  with 187 

chloridof. 197 

discrimination  of 197 

melting,  Deville's  furnace  for 190 

oxids  of 196 

preparing,  Wollaston's  method  of 187 

properties  of 192 

pure 191 

solvents  of 194 

welding 193 

Polishing-putty 260 

Purple  of  Cassius i49>  I5° 

Quartation  process  of  refining  gold 124 

Reduction  of  metals..- 109,  no,  in,  112,  113 


INDEX.  28l 

PAGE 

Refining  gold 124 

Reinsch's  test 115,  225 

Richmond's  alloy  for  crown- and  bridge-work 262 

Rolling  mills 93 

Ruby 244,  255 

Sapphire 244,  255 

Scorification 157 

Silicate  of  gold 153 

Silver 169 

alloys  of. 1S2 

chlorid  of 178 

compounds 177,   178 

cupellation  of 177 

deposition  of,  by  battery 186 

discrimination  of 17S 

estimation  of 179 

native 171 

nitrate  of..  177 

oxid  of 177 

precipitation  of,  by  copper 1S1 

precipitation  of,  by  iron 180 

precipitation  of,  by  sodium  chlorid 180 

precipitation  of,  by  zinc 181 

properties  of 169 

pure 180 

separation  of,  from  ores 171 

soldering 94,  95 

solders  for 184,  185 

solvents  of 170,  177 

spitting  of,  during  fusion 170 

sulfate  of 170 

sulfid  of 177 

Solders  for  aluminum .2^9,  254,  255 

gold 

silver 184,  185 

soft. 254 

Specific  gravity  of  alloys 38 

Speculum-metal 263 

19 


282  INDEX. 

PAGE 

Stannic  chlorid <  260 

Stannous  chlorid. 264 

Stannum  (tin) 260 

Steel....: 209 

aluminum 212 

Bessemer 211 

blistered 210 

cast 210 

chrome 212 

copper 212 

discrimination  of 216 

hardening 214,  215 

manganese 214 

nickel 213 

shear 210 

tempering 214,  215,  216 

tungsten. 211 

Sulfids,  metallic ....- 108 

reduction  of 112 

Supports  for  use  in  melting  and  soldering 97,  98,     99 

Tellurium ■ 17 

Tenacity 28,  39 

influence  of  alloying  on 39 

Tin,  alloys  of 260 

amalgamation  of 260 

chlorids  of 267 

discrimination  of 267 

estimation  of 66 

foil 265 

in  amalgams 5° 

oxid  of. 66 

pure,  preparation  of- 260 

refining . 264 

solvents  of. •  267 

Titanium • x9 

Type-metal 263 

Vermilion 223 


INDEX.  283 


PAGE 


Von  Eckart's  alloy  184 

Welding  properties  of  platinum 192 

Wire-drawing 93 

Wood's  metal. 263 

zinc 235 

alloys  in  dentistry 236 

alloys  of 236 

counter-dies 238,  239 

dies  for  swaging  plates 237 

discrimination  of 241 

expansibility  of 23 

in  amalgams 62,  64,  236 

'oxychlorid  of. 241 

properties  of. 235 


J~Z      <^J^.  C   ^-6  J^A^W^L, 


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